I only pan mono signals.
 
As was said above, stereo is already panned and normally it's set.... don't mess with it. You end up losing a side. If you really need it panned, convert to mono and pan away.
 
my 2 cents

Saw an interview with a producer that said he pans to the Audience perspective, but only after asking the drummer. It's not so important to the audience in general, but if it's going to mess with the drummers enjoyment of hearing their performance,  then go with the drummers perspective. Of course this doesn't matter so much if you are using BFD or something.
 
For Piano with stereo width, if you were to se to one side of the stage or the other, would you not place the bass end nearer the centre, whether be audience perspective or not?

Thanks for all the feedback.

In my case, I'm recording the drums with only 4 mics, due to limited inputs. One for kick, one for snare and two overheads, left and right, all mono. So I'm panning the overheads hard left / right.

Another thing to consider for your overhead mic set-up is that you have a number of options for mic placement or orientation of the two overheads.  Ultimately, the method of overhead mic’ing will come down to your recording space (i.e., the room), the style of music and how the drums sit in the mix.
 
You should probably experiment with different set-ups to get a feel for the different results.
 
- XY (coincident pair)
- ORTF 
- AB (spaced pair) 
- MS (mid-side) 
- Recorderman (1 mic overhead and 1 over the shoulder) 
- Gly Johns (1 mic overhead and 1 mic place horizontal on the floor tom side directed at the hihat) 
 
XY, ORTF, AB and MS all capture the room.  Recorderman uses a closer micing technique giving you a dryer sound.  You will have phase issues with ORTF, AB, Recorderman and Gly Johns.  MS requires a mic with a figure 8 pick-up pattern to capture the sides.
 
My preference is an AB mic configuration.  I know I will have to deal with potential phase issues but there is a fair amount of flexibility in how you aim your mic’s and I find that when I pan the tracks hard left and right it gives me a slightly exaggerated sound stage.

I hear this stuff about "from the drummer's perspective" or "from the audience perspective".....
 
Well, in 99% of the cases where I was in a club listening to the band..... as a member of the audience, you are generally far enough back from the drum kit that the sounds appear to be coming from the same spot. At most, you might have a 5 to 10 degree angle (or less)  from one side of the kit to the other, so for all intents and purposes it is a mono sound.
 
Seriously the only person in the room who hears the kit in stereo is the drummer because he is setting in the middle of it.
 
As a mix engineer/producer, we have the ability to make the kit sound in full stereo. That's a creative decision. I like the kit centered and if it's stereo from the source, I don't attempt to pan it. I generally accept the stereo spread that the track has and let it roll from that perspective.  If it's too wide and wild, I convert to mono......needless to say I don't ever recall having to do that.