I was listening to Sgt Peppers Lonely Hearts Club Band last night through my headphones and whoa!!!... there was some seriously hard vocal panning going on there!
Lot of stuff from the sixties had that hard left/right panning, which sounds pretty cool on most tunes, as long as it's done right.
It all depends on the type of song your recording, too.
You don't want to get too radical with a country song, love song, or ballad.
Listen to some of your favorite CD's through the speakers first, and then listen to the same CD's with a good set of headphones. This will give you a better perspective on just how hard certain instruments are panned.
Sgt. Pepper is an extreme example in hard-panning. The CD "Puzzle" by Dada is an engineering masterpiece worth listening to... it's one of the best sounding CD's I've ever heard.