For most Vegetable seeds I will typically look to keep the green house in the 85-90 degree range, you should get the quickest germination rate and the percentage of seeds germinating.
Part of the equation is how warm you keep the room where you're going to be doing this, Carol. If it's on the cool side, there's little chance of cooking the seeds. If you really want to optimize your growing conditions, however, a thermostat probably makes sense.
If you are not going to be there to monitor it yourself, I would invest in the thermostat. How big are you going here?
I have a digital thermostat (eight in fact).....my fingers! Noting Douglas' comments, when the soil is barely warm to my touch seeds seem to germinate best.
Walter, Love it!! You saved me some money.