I'm with rharv on this one. I always prefer the piano as a mono instrument - it sounds more natural and gives it more definition.

Also, I think Dan is right when he says that there are no fixed rules. If it sounds right to you, have the confidence to go with it.

The only other thing is that some pianos sound good on their own, but are often too dark to cut through a mix. Brightening the piano track can improve it's clarity without increasing the volume.

ROG.