The question comes from a discussion I had years ago with a keyboard player who had perfect pitch. His problem came from the fact that he lived in a very small appartment in an old building and he had to play at a very quiet level. So quiet that he could hear the 60hz humm (we are in north america) coming from electric appliances and that would bug him a lot. He thought about it a lot and he realized that the electricity itself was actually out of tune. Since most modern occidental music is based the reference not of A=440Hz, if you go 3 octaves below that, you get to 55Hz. But AC around the world is either 60Hz or 50Hz. When he made that calculation, he declared that 55Hz would be a very convenient and musical average if there is ever a worldwide standard.
So here is the question. Is there a reason for 50 or 60 Hz? Or it might as well be 55Hz?