I think there was a big topic on this somewhere else.
1.Firstly, it's damn hard, and
2.everyone has their own favourites
3. Every piano - even sometimes supposedly identical ones record differently
The piano has real width - so whatever stereo technique you decide on needs to be able to faithfully reproduce this width. If the piano is in a nice sounding space, then the old technique of putting the microphones away from the piano in the short string curve a little higher than the top, looking at the lid on full stick sounds quite nice - coincident pair, or some people prefer ORTF. I quite like a simple X/Y. M/S, as has been said is popular amongst those who like a bit of control over width after the recording, and for modern pop piano, of course a couple of mics closer to the strings works pretty well, giving you an artificial dryer sound - that you can always add reverb to afterwards.
So much depends on the music and the purpose. If the room is poor or just too small most mic positions tend to work against you. In a reverberant space, then you will need to experiment to get the placement right.
If you elect to go in close, you also get the percussive effect of the hammers and dampers thrashing around, and some pianists are very noisy with the pedals. Indeed, some pianos will have lost the softness of the pedal release dampers, and the damn things go clunk!
Shorter pianos with diagonal stringing often sound a bit odd close in - sort of 'jangleer' a bit more distance flattens this out. I record piano quite often, and like the sound of large diaphragm condensers in preference to the small ones as they are often a bit mellower, which seems to help.
Don't be afraid to experiment. My assistant once set up for a closer mic session, and accidentally got the front and back of the mics the wrong way around, and we recorded the bounce of the lid - and it actually sounded quite nice, with a rather gentle left to right width. So nice, that when we did another session with this pianist, we did it again on purpose this time. The books tell you certain techniques are best - don't be afraid to experiment.
Be aware that if you get your placement wrong, you can make people feel quite nauseous when the pianist goes from say left to right in a fast arpeggio as the sound sort of runs through your head - too much width.
I do have a rather nice stereo mic with a rotating top capsule, and this works in cardioid and fig-8 patterns, and in a super room, the fig-8 is lovely, but in a smaller or less live space, fig-8 sounds very odd.