When is it OK to wear a concert t-shirt?!

When is it OK to wear a concert t-shirt?

Here is a question for the ages... according to the "unwritten rules of cool", you are considered "THAT GUY" if you wear the t-shirt of the same band you go to see in concert, unless said t-shirt is purchased at said concert. You are also considered "THAT GUY" if you wear the t-shirt of a DIFFERENT band than the one you are seeing... so when is it cool to wear these concert t-shirts?? Why are you considered "THAT GUY" and why is someone going to, for example, a football game wearing the jersey of his favorite player NOT considered "THAT GUY"? don't both scenarios show fan support? Just curious...

I think it's cool whenever I go to concerts & I see people wearing the t-shirts of past shows or even better when they put the newest one on from that night - when I see people out & about wearing concert t's, sometimes I think how cool the t-shirt is or that I'd like to see them in concert or that I have seen them in concert & it brings back great memories - I'm a sucker for concert t's I guess....all for the love of music! - Next week when I see Gwen Stefani I'll have my t on from her last show....