It's not about whether or not you wear Cowboys gear... although if you don't and you keep your mouth shut walking to and from the stadium, you'll be safe no matter what.
The key is what situations you find yourself in.
Saying, "wear your stuff, but be respectful" won't cut it if you happen to find yourself alone, or badly outnumbered , or in the wrong place at the wrong time. Philly fans, will kick your arse, even if you keep your mouth shut, if they think they can get away with it.
If your friends wear their gear, then they are probably going in numbers (which is VERY smart) and they probably make sure they don't get isolated.
Howboutdemcowboys31... go, cheer for your team, and even wear your stuff if you want, but be very careful. If you wear Cowboys stuff, I'd bring a jacket or sweatshirt or something that you can wear as you're walking to and from the game... because that's where you're most likely to get jumped. At the game itself, with security people around, you're much less likely to endure anything worse than a cup of beer or a random punch at worst.
Have fun and go Cowboys!