Will that guarantee pretty much 100% you'll be a Bills fan? I was in Philly today and everyone had on Eagles gear. Okay sure it's easier to like a team that's just won a Super Bowl, but someone born and raised in Philly who grows up to be a Cowboys fans seems a little out of the ordinary. Plus Acme will give you discounts on your food groceries on game days if you wear Eagles gear to the store.
Growing up in NJ in the heart of Giants/Jets country, it doesn't guarantee you'll be a fan of either team. Those two teams for the most part are the most popular here but there's scatterings of other teams fans here as well. Cowboys, Dolphins, Steelers, and bandwagon Patriots fans are well represented here. Then I root for the Bills.
I imagine other teams in Buffalo aren't well represented fanwise...