I've had good results using both XY and ORTF stereo techniques on small choirs, usually from about 5 feet back and maybe 2 feet above thier heads. From you description, my biggest concern would be making sure the piano levels were right for the recording. To get the levels right might require an additional mic (or two) on the piano. If you're short on mics, you might be able to get away with one mic out front and one on the piano. Or, if the paino is loud enough, a stereo pair out front might be enough.
As far as phasing issues go, if you just use the stereo pair, it won't be a problem since both mics should be the same distance from the source. If you add any mics on the piano though, you should check for phasing problems during playback and fix any you notice. There are plenty of threads on here explaining how to do that by summing to mono, so if you need to, just search for it. I hope that helps, and good luck!
-Peter