It appears that during the ‘regular’ season there are only a handful of referees that still require a player’s shirt to be tucked in at the beginning of the match. Virtually all matches at higher levels do not seem to worry at all about this. I feel almost alone in this area – why do we still require this if so many (the majority) don’t give it a second thought?

USSF answer (April 5, 2011):

In the past custom, tradition, and safety required that players keep their shirts tucked in and their socks pulled up and generally maintain a professional appearance. However, nowadays the uniforms are cut differently by the manufacturers and the jerseys are clearly meant to be worn outside the shorts. It is time for us referees catch up with modern fashion and learn to live with it.