I do a lot of country music mixing, and I typically pan based on instrument. Here's the basic setup I use...
Drums - snare and kick in the center, hi-hat mid-left, ride mid-right, toms left to right in order of highest to lowest, and spread out the crashes as desired
Bass - center
Acoustic Rhythm Guitar (where applicable) - two ways, either record two rhythm parts and put one to the far left, the other to the far right, or just copy one track and spread those out, which ever you prefer
Electric Rhythm Guitar - same philosophy as above if no
acoustic guitar is used
Piano - if it is not essential to the overall sound of the song (i.e., it is not a piano-driven tune, like, for example, "Desperado,"), then I'd mix it to the right, not completely, but almost
Electric "riff" Guitar - If this is for additional licks, but no melody, I'd go to the left, same idea as piano, not all the way, but close
There is your rhythm track. Any lead vocals and solo parts (voice or instrumental) should be in the middle. If there are background vocals, I usually put the higher one to the left and the lower one to the right.
Like everyone else says, these are just based on preference, so you are open to experiment. I just hope this can be a help!!