Scott has a very good point. Listen at the round end of a grand, opposite the keyboard and you will hear a very wide three dimensional sound.
If you put a pair of mics beside the player's head, you will hear the sound like the player is hearing, plus a lot of noise from the nails against the keys, no good place.
There are so many placements possible, depends on what sound you're after, the room etc.
It's not only the mics that's important.