To elaborate on farview's post a bit, because the snare, kick, bass, and vocals all have distinct sounds and occupy different frequency ranges, you don't have to worry as much about them meshing together (with the possible exception of kick and bass, but even those should emphasize different frequencies). In terms of finding a distinct space for instruments, panning is often secondary to things like reverb and eq. Throwing the two guitar mics left and right is a good idea, because those will sound very similar and blend together no matter what, so the panning gives them a distinct space. However, panning the snare far left is not a good idea because nothing else really sounds like a snare, so it's not fighting with anything for space, and because snare is a crucial part of the song. If you're sitting on the right side of a car and can hardly hear the snare because it's way over on the left, the recording is going to sound unbalanced, whereas missing out on one guitar mic isn't going to affect the sound that much.
Panning is also useful for giving a realistic stereo representation of the instrument, which is why stereo overheads are cool for drums, and why I still do pan the toms going from left to right. But like farview said, not a hard pan, only 20-30%.
I do find it funny that bartman rightly pointed out that there's no right answer and we preceded to say: here's the way everyone does it, that's the way to do it. So remember these are guidelines, and you can do things differently if you're doing it for a reason, but we're just saying it might not sound as good.
