I think it also depends on if the tank has a center brace or not. An unbraced tank will bulge out slightly (I just built a canopy for a 33 gallon and measured - 36" long and 12" deep and it bulged 1/4" at the center). That would (I expect) slightly destabilize the tank for putting a heavy load on top of it. If you put a very large load over it, you might consider putting some thin legs on the canopy to let it hover right over the tank.
Right now I'm making a canopy for a reef that *is* center braced. I expect the thing will weigh a good 100 pounds when all is said and done (lighting, wood, plus a 10 gallon refugium on top) and I don't expect any problems. I'll report back if something explodes.