To summarize the rules...
#1 - Go with a buddy
#2 - It takes guts to suit up
#3 - Dress when it is likely that others will dress up
#4 - The bigger the expected crowds and/or box office, the greater likelihood that you should dress up.
And now, #5 - Only dress up when the movie first comes out. And I don't mean a week after the movie first comes out - I'm talking the first day and, preferably, the midnight showing. Because, let's be honest, any movie worth dressing up for is going to have a midnight showing.
Why is it more acceptable to go to the premier or opening day dressed up? Because that's the day when the uber-fans will be there. These are your people - the folks that have loved this property and have been counting down the days until the movie's release. They're much more likely to think it's cool that you dressed up for the movie than, say, Ma and Pa Stapleton who come to see the movie three weeks after its release.