With the exepction of Kappa, most people here are against them. I have seen them on girls in another sorority, but generally we do avoid it. My Big Big has sweatpants and her letters are on the leg and they look fine. I can't speak for anyone else, but with KLP we have a rule that you can't have anything with letters on the butt so as not to disrespect our letters and the meaning.
I personally think that's a great rule since I HATE HATE HATE butt letters.