Where I live is extremely hot all the time. To make it worse, we don't use A/C! So I don't have heaters or my fishies would fry! But, sometime during the winter (because we don't use heat, either) it can get chilly. So when those few cold weeks scattered around winter come along, I think I'll get one. That way I don't have to balance the temperature myself.
I think no matter where you live is useful. Yesterday was 100* but next week it can be 85*. You can't control the weather! LOL! But if your house always is set for the same temperature (around 82*-85*) then you should be okay. Anything colder than that will make your tanks cold. And no, being at 70* isn't going to *kill* them, it just weakens their immune system, makes them stressed out, and makes them lathargic.
The irony? The *hotter* it is outside the more you'll need a heater. Why? Because in most house holds that's when they kick on high A/C and it makes their house/apartment *colder* than during the winter!
Buy a $2 thermometer from WalMart and moniter the temp. If it's generally below 77* then I would get a heater.