I removed the heater from the 29-gal a while back. Decided it wasn't needed because we keep the house warm enough. Only have a heater in the betta tank now.
Except if you know the temp is going to be excessive and the heater won't run for an extended amount of time, why waste the electricity of having it plugged in? I know it won't run so it won't use that much, but I figure if things aren't in use, why have them plugged in?
If you just leave it plugged in the heater is not going to turn on unless the water gets colder than the temp it is set at.
On the other hand, if you keep fish that do well in room temperature, there is no point in using a heater at all.