In both of these cases, real life is not being emulated, it's an emulation of life (film) that's being emulated.
Pretty much hit it spot on. Most of these conventions (it seems) come from imitating sounds we've heard recorded on albums over the years, which came from producers trying to sound good/interesting, which may or may not have come from trying to imitate past albums... one of those circular things. It's quite annoying when the piano samples are panned that drastically though, especially in busy mixes where the piano isn't really a focal point. It's kinda fun to crank the volume and play solo piano stuff with hard panned samples, but when the overall side-to-side balance changes drastically just based on where the piano happens to be playing, blah.
As for guitars, there's plenty of difference between up and down strokes, in just about anybody. Course, there's also noticeable differences depending on how the rest of the phrase prior was picked/fretted too, but now we're just nitpicking. Thankfully I don't have to worry so much about that... I just pick up the guitar
I have enough headaches trying to go through all the sequencing tricks/hoops for everything else heh