I'm not sure about temperature limits, but I open mine during warm days when the sun is on them if they have sprouts. Don't want to cook the babies! If it's staying warm you can start hardening them off, and recover if the temperatures are going to drop sharply.

I was taught to always leave the lid off for transpiration. Maybe it depends on your containers. I use gallon milk jugs and 2 liter soda bottles. I'm starting to get a few babies here in Zone 5 and yes, it's unusually warm. Heidi

I believe she is referring to the top part of the container not the small screw on cap/lid of the 2L/jugs. Those caps could be recycled as are not used when winter sowing.

i open my containers with seedlings when temps are in the 70s and, as suggested above, close them if a freezing temperature is expected.

Winter sown plants do not need hardening off because they germinated outside and haven't been exposed to indoor temperatures.