Absolutely you can pan anything anywhere you want.
I would say that the standard for rock/metal would be to pan the guitars out and keep the bass in the middle.
Putting the bass off to one side can be pretty distracting, especially in headphones. An interesting technique I heard the other day in some Audioslave song was to take the high end of the bass (a separately treated track, more distorted and guitar-like) and put it off to one side opposite the guitar. Meanwhile the more low end bass track stayed in the middle.
Regarding drums, usually there is some stereo separation so not everything would be in the middle. Kick and snare are usually in the middle, toms are spread a little, and overheads are spread. Some people start by spreading the overhead mics and them placing the toms in the same location as they appear in the overheads.
I'd say just listen to some of your favorite bands in headphones. You may never know what mics, compressors, eq's, etc. were used, but panning is pretty obvious.