There's actually a very complicated algorithm for determining who I can or cannot root for. Without going into too much detail, it's okay to root for a team that has already won the Super Bowl, as long as there aren't any friends, family, or coworkers who root for them, and as long as it's not a team that I hate like NE*. It's not okay to root for a team that hasn't won it yet, because like you say, I don't want those teams to experience a SB victory before we do (damn you, Seahawks) -- unless they are playing against a team I hate like NE*.
It actually gets quite a bit more complicated than that, but you don't need to witness that level of my insanity.
Yeah -- they only talk about the Packers because they're good. Never heard about their fandom much before their latest rise, nor do they talk about them much unless they're winning.
I do find that strange, to be honest. Didn't think there were many Packer bandwagoners.