I live in the Bay Area but I don't travel much to San Francisco, so I can't answer your question of WiFi coverage.
But regarding tethering and having to Jailbreak your iPhone, you know that AT&T pro-rates the tethering plan, so if you don't want to pay for a whole months worth ($20), you can just pay $.66 a day and still be allowed to use up-to 4GB of data.
I gave up my unlimited plan because I never even came close to using 2GB, and even with tethering and my most heaviest usages, I still stayed under 1 GB (I was on a week long trip and shared my connection with my iPad and 2 other iPhones).