I had a similar question out of London:

http://www.flyertalk.com/forum/u-k-i...uber-zone.html

Short answer is it was fine, and London Uber drivers can sometimes go 100 miles outside of London.

So I'd say you're fine, especially as drivers can choose their jobs anyway. In the unlikely event you do have a problem, there are websites like Minicabit, and it'll only be slightly more expensive than Uber.