I just wanted to throw my 2 cents in on this too.
I thought Navy summed it up almost perfectly. I've been to a lot of games in other cities, and Seattle isn't that much different than the rest.
I haven't missed a game here since 97', and there are more of them, and more passionate fans now than years passed. But the opposition gets treated about the same depending on how the visitors act.
I typically always have a handful of visiting fans in my section. I will chat with them if they're cool, and are interested in some chit chat, and if they cheer for their team, and don't talk crap to the fans around them, I have never seen an issue in the 15 years I've been going to games. If they stand up and talk crap then the local fans pretty much incessantly yell at and taunt them until someone is leaving in front of a guy in a yellow jacket.
The bottom line is that this is the same at every stadium in every city I've been to. You see the same thing.
If you want to wear your SF gear, go to a fantastic stadium, and watch a great game, you should have a great time if you're willing to keep a lid on the smack talking.
If you want to go and talk smack, you have to deal with the crap that comes with it. This is no different than anywhere else.
All that being said, and one wild card that changes things somewhat is the game time. I HATE primetime games, especially Monday nighters. The traffic sucks, you get home crazy late, and have to deal with a ton of extra crap than you do with the typical Sunday afternooner (I guess I'm officially an old fuddy duddy). The drawback with this game is it is the home opener, against a division rival, on a Sunday night. This game above most others will have some super alcohol fueled fans, so there will be a much higher than normal percentage of drunks.
Sooooo, there is a chance this could get rowdier than normal, but like I said earlier, 9 times out of 10, if you keep it classy, you'll be treated with class.
If you want to talk crap, be ready for the baggage it comes with.