cooling off heats

I usually don't, i never thought it was a long enough time to cause any serious problems...

Avoid drastic temperature differences. If you unplug the heater, change the water (with water close to the temp of the tank), then plug it back in when the water is at least as high as the minimum water level on the heater, you'll be o.k. If the heater is on and not in water and is hit with cool water, it will break. If you can comfortably grab and hold the heater, it's cool enough to go back into your 70+ degree tank.

If it is a submerged heater then you will not have as much to worry about. As stated avoid drastic temp changes. I don't even turn mine off but mine is at the bottom of the tank. If it is one of the HOB heaters shut it off as when you drain water it will expose the heating element and it will get hot faster and can cause a problem. Most manufacturers say to wait 15 minutes on either end but if you put water back that is about the same temp you should be fine.