Sorry if this is over-simplistic, I'd rather appear patronising than not be understood at all lol.
Stereo imaging: Imagine your sitting in front of a full orchestra, say the violins are to the left and the trumpets to the right for example. Even if you close your eyes you should be able to pinpoint each instrument group within the orchestra (if you know what to listen for). The brain uses two mechanisms, delay and loudness difference. The frequencies from the right will arrive at the right ear about 1ms before the left ear (yes this time difference is detectable) and your brain uses the delay to localise the source of the sound. Also frequencies above about 1500Hz from the right will be much quieter at the left ear due to being blocked by your head.
(for more details on sound localisation see here: https://en.wikipedia.org/wiki/Sound_...uditory_system
When a performance is recorded, mastered, and then played back on your speakers, ideally the delay information should be perfectly preserved so that if you close your eyes, you can still tell which side of the orchestra the violins are on just as easily as if you were listening to a live performance. The biggest barrier to achieving this is usually room acoustics, reflections, standing waves, resonances etc which will not only give you uneven frequency response (easily remedied with acoustic treatment and room EQ), but also completely screw up all the delay information between the L/R signals that your brain uses to localise sound, and that is much harder to fix.
Inside a space as confined (and with as many sound-reflective surfaces) as a car or truck, you have almost no hope of maintaining the delay information present in the original stereo signal. This doesn't mean it can't sound good, just that you won't get the correct stereo imaging (ability to pinpoint where each sound is coming from like a live performance). In this situation, just use some trial and error with delays (and placement if you can) till you get it sounding 'good' and leave it at that. You could spend $$$ acoustically treating the inside of your truck but that's up to you.
Inside an average room, you're still going to struggle to maintain really accurate stereo imaging without some acoustic treatment, absorption panels, diffusers, basstraps etc. In terms of speaker placement, the ideal would be to have both left and right be at equal distances and equal angles from centre at your listening position, while also having both speakers and your listening position be symmetrical with respect to the room (this also requires a symmetical room layout). If you cannot achieve this you can add some delay to either to left or right channel to compensate but it will not be quite the same as having the ideal layout. The exact amount of delay you should add obviously depends quite specifically on how your setup differs from the ideal layout I described.
It also comes down to personal preference, some people require very accurate stereo imaging or they cannot enjoy the music, some people don't care at all, most (outside audiophile circles) aren't even aware the issue exists. Personally I can appreciate when someone has put the time and effort into a system to create great stereo imaging (that is I have pretty good hearing and can easily tell the difference) but it doesn't factor at all into my enjoyment of the music, I don't find a lack of correct stereo image to be at all distracting like some do.
For surround sound (5.1/7.1) all the same principles apply but the situation is much more complex because you've got 5 or more channels interacting with each other not just 2.
One thing I've found helps bass integration in my 5.1 setup is to add some delay to all channels except the sub (so theoretically the sound from the subs arrives first). This can improve the sound in most small (or untreated) rooms because of the room-gain (cabin effect) at lower frequencies. The enclosed space 'amplifies' low frequencies because of standing waves, so Audyssey turns down the bass for a flat response. However, the room-gain is caused by resonance, which means it will take a few milliseconds (the duration of one or two cycles/waves) for the energy to build up at that frequency and the sound to reach the correct level. To compensate for this I have the subwoofers start making their sounds a few milliseconds early by adding delay to all the other channels (actually I just increase the sub's distance setting on the AVR but the effect is the same).