Swatter,
Pete is correct about the rattlesnakes in MT being Prairie rattlers. The Western Diamondback is found in the southwest (TX, NM etc). I agree that, compared to diamondbacks and even Prairie rattlers, our Timber Rattlesnakes and copperheads have a mild disposition and their bites are much less serious. I suppose I can understand packing heat in some areas of the West although I still think it's ridiculous here in PA.
As for the Lewis and Clark travels, they were certainly in Prairie rattlesnake country. One of the members of the expedition was bitten by a venomous snake (almost certainly a Praire Rattlesnake) and I think Sacajewea treated him with a folk remedy and he survived. I think this was what Pete remembered.