Hello Mantid Z!
Now, heating is my speciality. I don't have any mantids to keep yet, though I have hundreds of ants, which require heating.
Now in my experience insects generally require anywhere from 75-85 F, but some more tropical species require higher temperatures. Make sure to check the species you will be hearings prefered temperature. Now if you think about it, naturally insects get their heat from above. (Sun.) They generally do not receive any heat from the ground. Insects become very uncomfortable when being heated from below. You can buy a heat pad with adhesive on the back, and stick it to the side of the container. Or even better, use a heating lamp to act as the "sun". Either way, I suggest not heating from below. I would assume it would not kill them, just make them uncomfortable. They would most likely adapt to heating from below, but it would likely require time.
To make it easier for you and your mantids, just heat from above using a heating lamp.
Regards
-Major