I once recorded grand piano with two condenser microphones (AKG c414) and due to the length of the arms of the microphone stands had the two diaphragms pointing past each other (in a cardioid shape). Someone much more experienced than me then told me that that is something you should never do, the diaphragms should always face away from each other. The recording I made sounded good, and I was not given any good reasons why my set up was not correct/preferred.
Any explanations as to why this is the case would be greatly appreciated.
Edit: the mics were placed at the top half of the strings close to the player. The mic on the performers right was facing the bass strings and the mic on the right was facing the treble strings, hence the diaphragms face each other slightly.