From what my understanding is on my campus the Pan Held council has it written you can't drink at any time while wearing letters and our IFC leaves it up to the individual houses to decide the policy they want to use. Im assuming alot of it depends on your national policy.

When I was a new member I was taught not to drink, smoke, or cuss in letters because it was "disrespectful". While I don't think it's disrespectful for those of age to drink responsibly in letters, most collegians are 18, 19, and 20. So I get it. A lot of them will drink illegally and in excess, so branding drinking in letters as "disrespectful" is another way to encourage members not to do it. I think campus culture has a lot to do with whether or not drinking in letters (or smoking/cursing) is acceptable or not. My greek system was small and conservative...none of the (sorority) chapters allowed drinking/smoking/cursing in letters, so when a member of any chapter did forget to take off their jersey before margarita night she got looks.

Since I'm an alumna on the same campus where I was a collegian I get weird looks if I go to dinner and have a drink while wearing a lavaliere or formal/sisterhood/greek week shirt. Drives me nuts.