the single most important step in setting up piano mics is probably going to be listening in mono. Getting rid of as much of the phase problems and comb filtering as possible by moving the mics around a few inches will really pay dividends, even when you go back to listening in stereo.
part of positioning will be determined by whatkind of sound you are going for, and what other instruments, if any, are in the room. if you need isolation, you might have to mic fairly closely with sound blankets covering the partially raised lid.
in general though, classical music typically is mic'ed from farther away - so you get a mellower tone with more room sound. this may not be a good thing if you hate the sound of the room. rock piano is usually recorded with close mics so you get a brighter more piercing sound, relying on artificial ambience if necessary.
Typically i end up somewhere in the middle. anywhere from one to two feet off the strings. I usually have one mic just above the strings about an octave or two from the top note (panned right) while the left mic is down closer to the toe of the piano, somewhere in the area of where the bass strings and midrange strings cross eachother.
then, spend a fair amount of time recording some snippets and listening back- seeing ifgood or bad things happen when youvmove the mics a few inches in various directions.
In the end, each grand may require a different scheme, and the room acoustics will also heavily influence your ultimate mic postions. also- don't be afraid to try out using just one mic. Mono piano recordings eliminate phase problems by definition, and can sound surprisingly good!
Hmmm . . .