For B and C tiers, and for most Am only tournies, no dress code is required, other than pretty much wearing clothes without rips or tears, etc. For A-tiers and higher, a collared shirt is required, but it can be a mock neck or if it's dri-fit material it can have any kind of collar. Most tournies you go to, no one is really going to care. Only really for big events will you find some sticklers about dress code. I find I play better if I feel that I look somewhat professional though. I usually wear cargo shorts and a dri-fit polo.
"a man is a success if he gets up in the morning and he gets to bed at night, and in between he does what he wants to do."
-- bob dylan