Just some things to consider regarding the "optimal level" point that David brought up.
1) if your overflow/drain pipe was to clog so that no water was running to the sump, how much water would be pumped to the tank before your return pump runs dry? Probably a good idea to set your tank level so that there is enough space to empty your sump to the pump intake level.
2) if your return line clogged or your return pump failed how much water would drain into the sump? Might be a good idea to adjust your drain position so it doesn't drain more water into the sump than it can handle. Also take measures to limit the amount of water that siphons back through the return line.
The chance of your overflow clogging completely to the point where absolutely no water is getting to the sump is probably pretty slim. Most people's setup usually will keep the pump wet even if its not moving any water back to the tank so it should stay relatively cool (at least cool enough to prevent burnout long enough for the reefer to notice the problem). Most pumps, once the intake line is about 1/4 to 1/3 exposed, will start sucking air to the point where they cant move water up the return line. But there is still quite a bit of water in there so you get that cyclical wet sucking sound. Most pumps will get hot in that situation but not hot enough for any permanent damage. Some will shut down if they over heat without any real serious damage. Some will just die completely. Most pumps (the ones you'll get at 90% of LFS and online anyway) wont do any damage to the tank when they die they just damage your wallet. 
