I hunted in San Angelo, Tx and the place was FULL of rattlers. For the most part, I heard them long before I ever saw them. When it's cool (early morning mostly), the snakes seem pretty sluggish.....and didn't rattle. Once it cools off to where the high is in the 70's, I don't think I saw any snakes at all.
I did have one REALLY close call, and short of knowing the snake was there, I couldn't have avoided it. He did buzz at me for a couple of seconds before he actually struck. Fortunately, my lab running up to check out the noise threw the snake off.
Good luck! Just keep your eyes and ears open, and you'll be fine. If you are really worried about it (to where you can't enjoy your time in the field), you can get the leggins from just about any sporting goods store.
A pissed off moccasin still scares me 10 time more than a mad rattler.