Glass cover- I set up this tank in November and have had it covered the entire time. I have not had any issues with temp during that time and it cuts down on the amount of evaporation. It is the standard glass top that comes with an Oceanic 30 cube. I know a lot of people don't like to cover their tanks, but many of these same tanks have salt spray on their bulbs, cutting down on the light just as much. To me, it is much easier to clean a top glass than to clean bulbs. It also helps from fish jumping out.
Evaporation- I evap about 2 gals a week, again much less than I would if the tank was not covered.
Cooling- Up until now I have not needed a chiller, again because I set this setup up in Nov. It is going to need a chiller, I know that. I have an iceprobe chiller that I will try to cool it with once it the temp starts creaping up. If that isn't powerful enough, I have a 1/15th hp Current chiller ready to utilize.