That depends. Are you saying your house is 80 degrees at all times, or its 80 outside? Where are you keeping them? They prefer temps in the 90s so they'll definitely be happier if they have a warmer area. But I have heard of dubias breeding at room temps, so it really depends how picky your group is.
If the room they are in is at 80+ at all times then no. no extra heating is needed. Mine are at 80-85 at all times with one 100w ceramic heatbulb and they breed like crazy. 85-90 is what most people say. Also make sure there is enough humidity in their bin as well or they will die while molting
if its a human heatpad, it can go under. If its one of those reptile or under tank heatpads any side would be the best. I have a human heatpad under on high, It doesnt raise the temp inside at all but it keeps the bottom nice and warm for them.