FWIW, I am 178 cm and ride a 54 cm frame typically. My issue with making a larger frame fit is that you'd need to have a shorter stem, which can make for twitchier steering. So all things being equal, I'd rather put a longer stem on a slightly small frame than a shorter stem on a slightly big frame.

(Also, I have fairly short legs proportionally, so the top tube of a 56 would be all up in my personal bits.)