I don't recall ever having taken a tripod on a plane, but your right, when I carry my tripod, in its case, I always think someone is going to think I'm carrying a rifle. And though the x-ray machine is the last place I'd be concerned (there it will look like a tripod, if not, they will open the bag and you're done) I would check my tripod, not carry it on. I carry on my sensitive camera gear because I don't want that be in the cold unpressurized cargo hold, but I wouldn't worry about a tripod if it was packed well. I used to always pack my skies in their bag surrounded by socks, and long underwear, I'd do something similar with a tripod. The only exception would be if I wasn't checking anything.