in the Tri-Cities region, and have very rarely experienced any problems w/ black flies (the gnats that bite).
The only biting flies I've encountered frequently are horse & deer flies, w/ deer flies being more prevalent. They'll attack me and follow me for miles on hikes through the woods if I'm not protected w/ a good bug repellent (30-40% DEET), and their bites are unpleasant. In general, though, biting flies tend to avoid sunny, open areas.
If you have a specific county in mind, it might not be a bad idea to contact the local Cooperative Extension office to inquire further:
Extension Service - Local Offices