Maybe I am lucky, but the only time I have had a clown jump, it went over the beck of the tank into the overflow box.
Now on the other hand, I have lost a couple of firefish to jumping... after the first one I cut a plexiglass cover for the tank, and the second one jumped when I had it off for feeding. And yes it does have an effect on evaporation, which I actually like not losing water so fast, and I don't have any temperature issues, and plenty of surface agitation and cascading from my filter for oxygen transfer.