IMHO, 'Fifties' is not requested because most of the time it's the younger crowd that does the requesting. The older folks 'hope' you play it and then they'll dance. It you don't play it, they'll just sit there. If you play for the young ones early on, they won't dance anyhow since they haven't had a chance to get liquored up yet.
I generally try to play for the older ones earlier on but throw in a newer tune so the younger crowd knows that I have what they like as well.