Among other things, it depends on the kind and quality of the microphone. Is not the same for a PZM than for a condenser or a cheap diaphragm mic. At first approach you can try by putting a mic where the audience is supposed to be. If happens that the accoustic of the room doesn´t help, then you can mic closer to the piano, and even open the lid, trying both the trebble and the bass.
HQ mic suppliers (Shure, Crown) have useful and free piano recording guides in their websites. Also you can do a "piano recording" search in this same forum, YouTube, etc.