It will also depend on the size of the nano is. I find larger ones do better with a heater and causes less restrictions with locations. As fishfreek
said though, a constant ambient room temperature and not being subject to drafts is very important. My ¾ gal
nano's do fine without heaters but the room temp is quite constant.
Best suggestion is once set up, run the tank for a few days and note the temp in the early AM, midday and just before the lights go off to see if it's needed before anything other than sand and LR