Mine are usually shorter than 18" at the 3rd leaf level, so I dont see much hollowness at that point, but I dont think you need to worry about water getting into the hollow stem and damaging them. Most experienced growers prune dahlias at the 2nd, 3rd or 4th leaf levels, and I've never heard anyone mention anything about needing to prevent water from getting into the stem there.

I have read that it can be a problem, that water, bugs, etc down the hollow stem can cause rot to the tuber. If its a very small opening I dont worry, but I have on occasion had a hollow stem with a 1/4 " hole or larger. I taped it over with electrical tape. I have used electrical tape for stems that develop vertical cracks too, it works well to keep them from splitting open. (looks a bit odd though)

I hate to waste anything. Will pinched off stems take root?

Wayne - the rule is to pinch them back right after that 4th leaf set begins, so what you are pinching of is only about an inch or less. I have never tried to root Dahlia tops, I doubt that hollow stem cuttings would root, if small enough to be solid , then possibly, but with Dahlias, its easier to just wait and divide the tubers.