I do that. I've been using the heat vent to germinate seeds and cuttings this winter. So far so good. Does your heat get turned down at night, like it is recommended?

In nature days are generally warmer than nights and your heating being shut off or turned down at night shouldn't hurt your chances of germination.

For germinating seeds it does not matter the source of the heat as long as the soil reaches into the range of temperatures required for germination of a particular seed. Some seeds will germinate better if the temperature fluctuates as it does in nature. Al

I have an old freezer that I put my seeds on top of. Worked great last year.
Duane

I've used floor registers or radiators as bottom heat for years. I've found it to work quite nicely.