I used to use stocked drums presets (Session Drummer 3 / BFD 3) as they are, except perhaps tuning the high / low pitch a little.
 
Recently, I was recording a real drummer and mix it with the drummer's perspective - snare/hi hit more on the left, with the hi to low toms spread towards the right - using most piano sound as a guide (low notes on the left, high notes on the right).
 
So, that got me to pay more attention and on my current project, I found the BFD 3 preset I'm working with is panned the opposite - hi hat towards the right, floor tom on the left, from the audience's perspective.
 
There's probably no rule on this. But how would you like to mix it ? Both drums and piano - player's or audience's perspective ?

I pan drums from the audience perspective.  However, how wide depends on the style of music.  For big rock drums I pan them as wide as I can get.  I usually pan the kick and snare straight up the center and spread the toms right to left.  For jazz, I treat the kit as a whole and keep it a little narrower--including overheads.
 
I don't usually pan a piano across the speakers.  It's usually either to the right or the left (depending on how I envision the "stage").  If there are no other instruments besides the piano I may pan it more center and then use some reverb to widen it a bit.  From the piano players point of view the lower register is on the left and the upper register is on the right but that's not how a piano sounds to someone listening.  So if you're going for a natural--realistic--sound then it's all about how it sounds in the room.
 
My approach to mixing is always from an audience perspective.  It's very rare for an audience to be sitting at the piano or at the drums (or any instrument) and even less likely for a recording.  From the audience's point of view, each individual instrument is a mono sound source that may occupy a different space on the stage.  So, I move things around by panning them and placing them on a "stage" in the stereo field.  When it comes to effects like reverb I decide how big I want the room and pan it appropriately.  I might use some reflections from the opposite stereo side to get a sense of width. Even an orchestra can be thought of like this; ie.  the 1st violins are no the audience left, the cellos are on the right, etc.  But the 1st violins may reflect around the room, same is true for cellos.  This, to me, is where the illusion of "stereo" comes from.
 
This is less true for electronic music.  You can spread synth pads really wide because it's not a natural sound.  You also might have the concept to put the listener in the music and you mix from stage center instead of in the audience.
 
Then again ... you can do anything you want and as long as it sounds good (not necessarily "right" or "correct") then go for it.

It's a personal preference. I prefer the audience perspective, hats on the right. But even that's not carved in stone - you could, for example, have a left-handed drummer who puts the hats to his right.
 
But I'm not consistent about it: I always use the player's perspective for piano. Maybe it's because I'm a piano player and not a drummer.

I mix drums from the audience perspective but when recording, I give the drum his mix with his perspective.
 
For keys, I am with bit on the player's perspective.
 
Vanblah has good advice on panning as well.  It can be rewarding to note the drum panning in all kinds of different genres of music to see what great producers used.  

panning
 
does not equal
 
stereo
 
 
panning mono signals left and right, is mono panning.
 
having a stereo signal, means it was captured with a stereo mic setup.... and the phase of those two mics in relation to each other is CRITICAL
 
do not mess with it.
 

It doesn't matter - except for drummers.  And whether they want to play the part or just listen ;-)

It's funny because as a guitar player and a n00b engineer I had never thought about this type of thing until I really started getting into mixing concepts. I did play drums for a few years shortly before I started taking the studio thing seriously so I was used to both perspectives but after lots of screwing around with panning on drums I realized that despite having been behind a drum kit regularly that the audience perspective sounded far more normal and natural. It seems to be what is usually used (as far as I can tell) and you gotta figure, what's a fan gonna hear when they go see the band live? The "audience" perspective. Of course, as was mentioned, there could be a leftie drummer (in which case I guess it would be appropriate to switch things up so the recording reflects a live performance) and perhaps club sound guys might go for drummer perspective (or be so used to panning the drums to a rightie setup a lefty becomes a rightie through the PA) that... well... that makes my brain hurt.
 
Personally I think audience perspective is probably best if your going for a "live" feel from a "live" band. I'm a dinosaur at heart though so whatever.
 
Piano? Damn... that seems tougher because you've got so many possibilities there. A standup against a wall with the audience behind, a grand kind of sideways on stage, etc. I think I'd use the pianist's perspective. When I listen to piano music I kind of like envisioning myself playing it even though I've never really played much piano at all. Gotta learn though. It's one of my musical goals in life.

For piano, the long held traditional view is that you must pan it from the audience perspective.  Which is a little awkward because the string have a front to back alignment from the audience not left to right.  For the most part however, if you are miking outside the piano with a stereo pair, the mic pointing to the right will be panned right and the one pointing to the left will be panned left.  Truth be told, if you listen to a classical piano album, you will not hear much of the separation between bass and high strings, since the mics are usually placed several feet outside the instrument in a great sounding room.  You just hear the wonderful sonority of the piano in a spacious environment.  
 
Personally, as a piano player, I am also used to the sound of the player perspective, and in pop music, depending on the needs of the mix, bass on the left is perfectly acceptable, and I sometimes prefer it if a wide stereo separation needs to be made.  

Thanks for all the info and good advise ! 
 
Lot's of experiment to try out to keep projects fresh.

I think audience perception can be almost anything depending....so I tend to mix  with enough separation until it sounds good. I don't often think of the, "where they might be sitting if this were real" thing. If I mixed orchestra it would be more critical I suppose.
 
I tend to pan pianos and drums based more on where I want them in the mix which is habitually bass left because I am accustomed to hearing it there. With drums, since I'm not a drummer, I pan toms and cymbals either way. I don't often pan more than  10/20% either way so it sounds good in stereo but you don't loose much if you only hear one side or in mono.
 
Bat has a good point...the combination of mono channels can give a stereo effect at the master, but stereo channels are usually already in stereo and if it was done right it is already ok.