That a bit of heat jump starts growth is very evident with the Tomato seed which I sowed on Tuesday. By yesterday a few seeds had started to poke their cotyledons above the surface and overnight most of their brethren have decided to join them.
The problem with having the heated propagator on a windowsill is that the sunlight is coming from only one direction and therefore the seedlings will grow towards the light. Some people will place a mirror or even white polystyrene on the room side to bounce the light back and facilitate upward growth. That seems a bit of an effort so I will just follow my usual practice of rotating the trays every day until such times as I take the them out to the greenhouse (unheated).