My tank is between 78-80. As said above, I don't start worrying until its a few degrees higher.
77 Is perfect, don't fret it. Your heater might be set lower but either A) It is lying to you about where it is actually heating to, or B) Ambient temperature is keeping it around that mark, and other things are increasing it.
I have one heater that is set to 78 and it kicks on higher then that, and I have another set to 80 that kicks on lower then that. This is partly why I am in the camp of having a heater controller to make sure they shut off if temperature rises too high.
Heaters, powerheads, pumps, reactors, lights, and pretty much anything you plug in and then put in the water will add heat to your aquarium.
One time I forgot to plug in my heater-controller and left my heaters off overnight, with my apartment AC on full blast, and when I woke up the temperature had fallen from 78 to 74 F. Apartment ambient was something like 68-70. (I try to keep it colder, but the damn building is too hot). The powerheads/pump/skimmer/reactor in my tank kept the temperature up.