I've used a heating mat, but I found, now that I have banks of fluorescents which I use for my seedlings, that the seeds on the higher shelves are sufficiently warm for good germination. I still use the heating mat on the lowest shelf.
Before that, I used to use the top of my water heater (with some insulating material in between), which was quite effective, although I had to check regularly.
I do put "warm start" seedlings on the upper shelves and "cool" ones on the lower shelf of my unit, also... but that doesn't quite get me to the 80 degrees of a heat mat. Less warmth doesn't mean you won't be successful, it'll just take a bit longer. I'm always amazed at how quickly seeds germinate with just a little more heat.
I find I check my seeds regularly whether or not I "have to." LOL