This is very subjective and will vary depending on the piano, player, type of music, and the room you're in. You'll notice I used the mics in spaced pairs rather than in an x-y configuration. This is because the x-y technique works best with smaller capsule microphones. Because of the capsule geometry in larger diaphragm mics, the off axis response can be boomy in the cardioid pattern. If you use the x-y you're exposing this off axis side of the mic to the center of the instrument.
Spaced pairs is a better way to go with large diaphragm mics in cardioid pattern.
Once we decided on the mic position we experimented with switching the patterns on the mic. We went from full omni to cardioid, passing super and hyper cardioid along the way. We recorded a bit of each and then listened back. We decided that a wide cardioid was the best sound. Keep your eyes on this page for further and new developments.