Gittergreen, I think that seedlings are much sturdier than that actually.
I used to use heat under them but I don't anymore, and honestly I think that there are more
potential perils with heat vs. benefits, as the heat can dry the medium/paper-towel/whatever a little quicker, cooking the roots/seedling (if you're not careful enough, obviously you take smart precautions). I realize that you are adding very little, and controlled warmth. I used to do similar with towels over the heating pad for subtle heat.
I stopped the practice of any heat under seedlings after reading an article/post by Subcool on germinating, in which he called heating pads the biggest enemy to seed germination. In it, he also talked about some seeds he spilled near his BBQ grill when sorting them. He eventually forgot about them until he noticed them growing up through the gravel near his grill, in 40-50f temperatures outside (something like that). Healthy as ever, and he had pics to prove it. Also, seeds sprout all the time outside in temps that are very low compared to what we ever give them.
Since letting go of the idea myself (warmth/heat for seedlings), I've since germinated seeds primarily in a room that is about 67f, with no heat added at all. No probs, and they germinated just as quickly. Granted, once they are up and going, I move them to another room with more optimal warmth/light. But... think about nature and the brutal unpredictable environments they've faced for generations. Me offering up 67f with no heat under is paradise in comparison, ya know?
I worry more about temps/consistency for cloning, personally, though maybe that's not as big of an issue as I think, either. That's the biggest reason that I do some flouros 24/0, for a consistent cloning environment.
Please don't think I was trying to disrespect your opinion, I just wanted to add some food for thought, as I believe that seedlings aren't nearly as fragile as we make them out to be.