We have glass on our tanks and clean it weekly during our water change and maintenance regime. Glass is barely dirty, but it does prevent ANY water from splashing up onto lighting, which is much more of a PITA to clean than glass. Powerheads at the water's surface, so there is a lot of surface agitation. Overheating has never been a problem, but then, we don't have a canopy per se, only lighting fixtures similar to this one raised off the glass a couple of inches:

http://www.jlaquatics.com/cgi-bin/sh...logno=zhm-hd31

If your canopy is open in the back, has fans to remove the hot air, I don't see how overheating is going to be a problem.