So I've got an Onkyo SR805 on the way to me as we speak and i have a pair of B&W 602 series 2 front speakers taht are bi-ampable.

SInce the Onkyo will bi-amp and the speakers will take it, is there a good reason not to? (center channel not being bi-amped?)

What kind of cables would you use for the bi-amping. I'm leaning towards getting the bluejeans made biamp as it can be had for about $60 for two 6ft lengths.

I have 2 good reasons against bi-amping:
You cant use back surrounds, and bi-amping only makes wannabe audiophiles feel good, it doesn't cotribute to noticeable improvements when dealing in receivers.
Bi-amplification only is beneficial when using four monoblock amplifires on stereo installations.

Well I don't have back surround speakers yet, and I probably won't for quite some time (if ever).

And how could more wattage not be a benefit? 260w is better than 130w of the same quality power.

Except that you won't be sending 260w to the speakers, you'll be sending 130w to the woofer section and 130w to the tweeter section. Different animals. What you're proposing is known as 'passive bi-amping'. A true biamp would be as fanboyz says above (though you wouldn't HAVE to use monoblock amps, you could do it with a pair of stereo amps), with electronic crossovers between each amp and your preamp.

Now, there's no reason why you can't biamp your mains using the back channels of your receiver, esp. if you're not going to be using back surrounds anyways. However, its sonic benefits are subtle at best. Using this method, you aren't doubling the amount of power to your speakers.

I do understand most of that, but let me try to rephrase what you said how I understanded it to see if we are now both on the same page.

biamping sends discrete signals to the tweeter and woofer.

This signals will both be passing through their respective crossovers inside the speaker (currently the way the tweeters get their signal is a bar connecting the two terminals).

There is minimal benefit bi-amping in this configuration because the tweeter takes minimal wattage and the wattage to the woofer therefore is not increased substantially.

The way biamping can be very effecitve is if the crossover is placed before the amplifier stage therefore the amplifierers are only amplifing the signals that will be reproduced by the speaker.

im buying a pair or neuphonix speakers with bi amp and a rotel RB1080 for the LP and a rotel RB50 for HP

I've posted this before but it bears repeating. Bi-wiring is different electrically than "mono"-wiring but doesn't produce an audible difference in sound. "Passive" bi-amplification is just powered bi-wiring. The only possible benefit would be adding more amplifer power and that is true only if the existing amplifer power is inadequate for whatever reason. Since all the amplifiers in the A/V receiver share a common power supply, the power supply itself is the limiting factor, not the number of amplifiers. So you aren't really adding amplifier power. You are just dividing the power supply's current to more amplifiers. You're just adding wires and changing the electrical characteristics of the connection without making a change in sonic performance.

True bi-amplification requires installing an active crossover between the preamp and a pair of separate stereo amps, COMPLETELY DISCONNECTING the speaker systems' internal crossover networks, and then wiring a separate stereo amp to each pair of drivers or to driver arrays. True bi-amplification is quite different from what most consumers think it is.

Using an unused pair of receiver amplifiers in the popular fashion of passive bi-amplification doesn't and can't make an audible improvement. It can only provide a pastime for the believer.

I know you dont have to use monoblocks.
But if your gonna be extreme be EXtreme 2 the DIVAMAX!!!

Read THIS (http://www.avsforum.com/avs-vb/showthread.php?t=987279) recent thread. All you need to know, and more.

(It was only a page back, did you even look for threads regarding this subject?)