As a 501st member I don't care, so long as whatever you're doing in your costume doesn't affect me. You're just like everyone else at a Con. I treat everyone with a certain degree of mutual respect, so don't take my comment as a negative. There are, however, people that I've come across who were not members of the 501st, who'd do very questionable things in their personal star wars costume only to have it come back to US, the 501st, as being the "culprit" for their actions.
Case in point?
We had a Embassy Suites that we used during a major convention in Orlando. There's another costuming group, the "wolf pack," which is one of those "all-inclusive" exclusive clubs. They had a huge party at this hotel, resulting in some significant complaints, damage IIRC, etc.. Who got banned from ever using the hotel ever again? The 501st did.
Mind you, this hotel, our garrison has held great relationship with this specific hotel that they'd bend over backwards for us. What happens? Another costume group that people mistook for the 501st gave us the repercussions of other people's actions.