I mostly use stereo reverbs, so there's no panning...it's just a full L/R stereo image.
Ditto.
But as Miroslav says, there's no hard and fast rule-it depends on what sounds right on each individual case.
However, think of it this way. If you're standing in a large, reverberant room and shouted, you r voice would come from a single point but the reverb seems to come from all around you. That's why anything other than Stereo L/R is a "special effect" not a natural sound.
This is something I've often wondered about...I try to use a couple 'verbs per mix, sending each instrument (guitars, vocals, snare, etc) to 'em at different levels...
Sometimes I'll pan the send on the guitars to make it be on the opposite side (guitar panned 100L, then I'll pan the 'verb send 100 to the right for example)...I know there are really no rules, but again, I've often wondered how other people do this sorta thing...