Heat mats are generally just for germination. Too much heat once sprouted only results in too rapid growth and long, leggy plants - not something desired. Same goes for the humidity covers.

You'll find several related discussions on proper use of heat mats over on the Growing from Seed forum if you want to review them.

Dave

Once germinated, soil temp should be between 60 and 70 F. Legginess is do to low light levels. Keep fluorescent lights no more 6 in. from the top of plants.

While I agree that legginess is predominately due to low light levels, excess heat can also cause plants to grow too rapidly - specifically tomatoes - and since most of the seedlings benefit from slower growth, air temps kept in the 55 to 65 degree range in the greenhouse work best for us.

Dave