When in the woods, I do keep a vigilant eye out for copperheads. The dangerous thing about them is they don't have a slither off instinct, they tend to freeze and in some environments blend in well enough that you can step on them. Getting bit by one doesn't mean they'll envenomate you, as poisonous snakes don't like to waste venom on something they can't eat. But the younger they are, the better chance of them squirting venom into you. The young ones are always the stupid ones.
I saw a nature show in which they did some tests on copperheads. What happened was that just stepping on a copperhead
usually didn't annoy it sufficiently. To really get one to bite you had to try to pick it up.
That didn't help one of my barefooting friends, though, when he stepped on one (while bushwhacking in southeastern Ohio). It did bite him, and he spent the night in the hospital (with quite a bit of pain).
I'd also note that "a vigilant eye" doesn't work all that well for copperheads, since they have such good camouflage.