FWIW, my experience. I put a 20g nano on my desk at work. Went topless at first - made it a lot easier to feed the tank, do water changes, etc. My PC Coralife fixture is on legs attached to the edge of the tank and futzing with a top every time I wanted to get in there would have been a pain.
I switched to a top to cut down on evaporation. I was losing several gallons a week. That's major evap from a 20g. After I put the top on, I'm hardly evaporating anything at all now. I've gone through maybe 1/3 of a gallon in a week. Since I don't have an auto top-off setup yet and have to carry in all of my RO/DI water from home, this is quite a nice improvement.
Temperature wise, I went up 0.3 degrees. Almost no difference at all.
I miss being able to just throw food in whenever the clown swims over by my keyboard - now I have to flip the top up which takes a second due to the lighting arrangement - but the lack of evaporation is very, very nice.
