I should check in here. im a 501st member and went to a con in my sandtrooper, but I just walked around and didn't go to hang with my legion brothers. I paid for my own ticket. had I accepted a 'vendor pass' then id have been obligated to stand by the backdrop for pictures. I didn't get any gripe from my legion buddys for not hanging with them, I was there at my own volition and theres no problem wearing a legion suit around casually.
on the other side, I also made my friend a kanan jarrus costume, it wasn't my best work but everyone who watched rebels knew who he was. he said that a rebel legion member approached him wanting to join since their squad doesn't have a kanan, but like others who have posted here, that member had almost immediately belittled the costume for being made of foam and the slacks having pockets etc.
my advice whether 501st or rebel legion members, please don't be rude to blatently come down on someones work UNLESS the wearer actively asks "would this be good enough to join" etc questions, but that should be common sense. And to the general public who wears the rubies boba fett/stormtrooper/vader who we know are inaccurate costumes, there aren't any rules saying you cant wear that. cons are about community and presenting your works as such. just focus on you, and have enjoy yourself.