As far as I can see, Ubercart allows rules to be established (through Workflow-ng) but they only go as deep as the state level, no counties or ZIPs.

Interestingly enough, I was exploring some Florida shopping sites last night to see how they handle it. I'm in Hillsborough County so I would expect to 6% Florida sales tax and 1.5% Hillsborough County discretionary surtax. One site I visited (which was located in Gainesville), was charging me about 6.22%. So, practically speaking I'm not sure that other online stores are following the rules anyhow.