I don't use a heating mat, but I start my seeds on tables, covered by a plastic sheet, within a heated room (75F). The sheet prevents evaporative cooling of the soil. I remove it after they are all sprouted to prevent mold, but if you look at my grow blog, you'll see they are growing fine. The room is also humidified to prevent this evaporation. The fact they are on tables ensures the bottom of the trays are just as warm as the room, rather than the floor. 9 types of tobacco germinated in a week or less, while two took two weeks, and one took two and a half weeks. I had one batch of tomatoes, and peanuts in there, and they germinated in 4 to 6 days. However, another batch of tomatoes, eggplant and peppers which were instead in a corner with two windows, facing south and east respectively, on the floor, have not shown any life in 6 days so far. I have a heating mat, but I fear from the one time I used it that it is too hot. It got up to mid 80s. A thermostat would help. I'm using my heating mat outside in a cold frame with my onions.
