I leave the tops on until they sprout, bec. leaving them covered and warm, they should sprout sooner.
Once you have sprouts - it depends on what you have - for example zinnia is tender, frost will kill it. So I open them during the day when the temps rise, and cover them at nite or when the temp dips toward or below the frost line.

you could alternatively increase the number of vents. More sprouts die from being cooked than being frozen. I don't know what temps are in yr zone now, but temps inside the little 'greenhouse' can be 20 degrees higher than outside.

Tops on to keep rain from splashing out the seeds, keeps out the critters, helps keep the soil moist, but put them in the shade so they don't cook. Once they sprout then the tops can come off and the containers can be set out in the sun.

Karen

Also realize that if you take the tops off, you will have to watch them closer because they dry out quicker. I, too, always struggle with this decision. This year, because it has been so cool, most tops are still on. But in other years, I've fried seedlings because what looked like a cool day when I left for work in the morning turned into a warm day in the end....

Bottom line - -weigh the pros and cons for your temps/lifestyle (do you have time to take tops on and off? water?)

Thanks everyone for your helpful replies. I peeked this morning and the Peppermint Zinnia's are starting to sprout...Ithought the Cosmo's would be first...weird. I wonder if I should have barely covered them with soil instead of just pressing them into the soil?