As noted by a couple of other electrical engineers, offering their valuable services FOR FREE after having spent many hours and thousands of dollars studying, people are going to gain nothing in audible sound quality by using anything larger than 16 gauge wire for such runs. It is not a matter of opinion, it is very basic science, and no one can rationally argue with very basic science without good reason. Mathematics does not lie, unlike salesmen.
If you are unwilling to get the degree for yourself then at least do yourself the favor of respecting the opinion of those who have. While the absolute losses of 16 gauge wire in your application are real, they are not just negligible, they are scientifically and experientially proven to be completely inaudible. For example, the losses of the recommended wire are on the same order of magnitude as the combined irreducible losses in the printed circuit board traces, the connectors, internal wiring, discrete components, and even the voice coils of the drivers themselves. The entire audio system is designed with these losses in mind and they are negligible. Reducing the one loss component in the wire between amp and speaker below the loss in the rest of the total system will do nothing at all to improve the sound.
I am sorry, this is about the best explanation anyone can give and more than most will offer a layman. Even the inductors, capacitors, and resistors in the crossover network of your speaker are not going to be constructed of, or connected with, 12 gauge wire, nor would anything increase if their wire gauge were increased, other than the cost. If you need proof, pull the woofer out of a cabinet and take a look at the internal wiring of your speaker. You will find a 'relatively' substantial amount of electrical loss inside there, unless the company is in the business of selling prestige at a premium price with no associated value other than bragging rights, in which case, cost is no object to you. Then you could safely ignore all advice and just buy whatever makes you happy regardless of whether it actually improves the sound or not. Probably you would also end up with a system that costs more and sounds worse, because once marketing trumps science, all bets are off.
The capacitance, inductance, and resistance of speaker wire is nowhere near enough to appreciably affect the frequency response or any other characteristic of audio except in extreme circumstances, and even then, the first thing to suffer audibly is the power, not the frequency response. Only if you are driving ridiculously low impedance and/or ill-behaved loads, such as White Van speakers that sound terrible anyway, or incorrectly paralleling up gangs of individual drivers on a single channel, will you notice any change in sound quality as opposed to quantity.
Even if using extremely long wires (greater than 100 feet), and experiencing substantial attenuation combined with perhaps some slight high frequency loss, you still are unlikely to notice anything at all except that you have to turn up the volume a little more (particularly on the channels with the longest runs), unless you habitually listen at extreme levels all the time and the dynamic range of your amplifier is substantially inadequate for your system. If your wire gauge were changed by one grade (from 16 to 18) without your being told, you might not even notice that anything at all has changed, especially if your receiver is the type that automatically adjusts the level, timing, and equalization of your speakers during a self-calibration.
Without self-calibration feature (or equivalent), or without re-running the calibration after changing wire size, you might notice a tiny shift in the loudness equalization curve (if you use it), i.e. there might be very slight relative attenuation of bass and (more likely) treble because the overall gain of the equalization circuit might be set a tiny bit low, but then again, depending on your speaker sensitivity and frequency response, a reduction in gain might even be an improvement!
This is all compounded by the fact that all of us experience high frequency loss with age. The effect of such changes becomes less and less noticeable as we grow older, yet paradoxically, more money is spent chasing the extremes of high frequency response by the aged, even though they have no hope of ever hearing it.
The only additional consideration one needs to take into account is the absolute current passed by the wire. If the gauge is too thin, the wire might burn like a fuse, and that could be very dangerous, but only if the system is driving lots of power through low impedance speakers. I am too lazy to do the calculation and leave that an exercise to the reader. All I can say is that my own 100WPC Onkyo home-theater-in-a-box that I bought cheap off Craigslist comes with 18 gauge wire and no company that meets UL listing would ever risk burning down a house with inadequate speaker wire.
So, if that is not enough to convince you, then the only alternative is to get an education, or overspend on your speaker wire, unless one of the factors below, or some other rational basis, exists for ignoring this rule of thumb you have been given.
Things to keep in mind relating to wire gauge and cable management, are:
1) Smaller gauge wire is easier to route, stuff through conduit, and hide
2) Smaller gauge wire is less expensive, saving cash for other things like conduit, paint, staples, popcorn, and videos
2) Larger gauge wire is more resistant to vacuum cleaners, pets, foot traffic, and accidents
3) Larger gauge wire is easier to extend if future expansion puts you in a situation where you might have to pull new wire later
4) There are laws and codes that govern speaker wire inside walls, and you should familiarize yourself with them
My personal rule of thumb, tailored specifically for my transient lifestyle, is to use 14 gauge for all my runs simply because it is more durable and I frequently re-arrange my system. That leaves me the flexibility to connect together cut pieces for extra long runs if needed and helps prevent my kitty from chewing clear through my speaker wires (yes this has already happened several times to me, with several different kittens). If you have a cat and your runs are exposed you might consider 14 gauge also, or even 12 gauge, with some rational justification, otherwise you could safely use 16 gauge.
Given that I now live in an apartment and cannot utilize my system to its full potential anyway, my 14 gauge wire is definitely overkill. Like I said, it keeps my kitty from sawing clean through it, but I have no other basis for the choice.
I have personally witnessed 'monster' systems with 8 gauge speaker wire in 10 foot lengths connected to vacuum tube amplifiers. The output transformers of these amplifiers have more orders of magnitude more damping loss than the wire. It is frankly ridiculous, but it looks quite impressive running along the floor. If one's objective is to trip your guests and make a fool of oneself before anyone with common sense, go for it!
Sorry, this is the absolute best answer I can give you. I hope it helps you understand why you got the other answers that you did.