I don't unplug my heater during a water change because once the water level starts to get lower than the heater I just slide it down into the part of the tank that has water in it.
Nope never done this... I've stuck my hands into many many many heated tanks (not just my own) and have never been shocked.
My own tanks heater never reaches above the water level when I do a change so the only way it could crack are 1. some freak accident 2. the fish got vicious and somehow broke it. Both are highly unlikely.
I never unplug mine either but most of the time it hasn't been heating up, it's been too hot here. Now that it's winter and it will be running more, I probably should but I won't remember to.
There is a differece between saftey and paranoiya, you know?
Not that i'm saying your paranoid....But I have had a hood colaps on me while my hands were in the tank - The light was on and everything. Pulled my hands out quick and riped the light cord outta' the wall. Pulled the actual light out by the cord...the fish were ok, the light still worked. I wasn't electricuted.