I have an open top nano and the temperature drops down when my halides aren't on. For me, heaters are a must, but even if they weren't, I'd use one just in case. To me, the probability of a heater failing is less than the chance of something happening that would reduce your tank's temperatures, like having one of the powerheads that help heat your tank shut off/fail, or waking to a really cold morning where your house heater didn't kick in.
If you're worried about a heater cracking, get a titanium one. If space is an issue, check out the Hydor Thermopump, a combo powerhead and heater. I currently have one in my nano and it works great.
Just my 2 cents ^_^