Fans aren't a fundamental requirement. I don't use them at all. So... whenever you want to.

I would say, leave them on. There are studies that show that anything that stresses the stems will cause stronger, more robust stems. One even showed that the best plants are produced on the aisles of a greenhouse because the caretakers brush the plants going by them. That's exactly what you want in seedlings. In my opinion, high light, low water, minimal temps and fans make great seedlings.
Rick

I agree. If you can always keep some good air flow around any indoor plant. The breeze is what makes them do the work to grow thicker stronger stems, and that makes for healthier plants because thicker stems can support more fluid movement.

If you're growing seedlings to transplant you need strong plants because the transition to outdoors is hard enough on them even if they're very strong. If their stems are weak they have a really hard time adapting.

Optimum temperature depends on the plants, the best thing to do is to look at the temperature range for each plant and try to find the coldest temperature they will all tolerate. Then maybe just a bit warmer than that. A few degrees above minimum, just so you can avoid getting too cold.

Internal air circulation is beneficial, for many reasons,
including inhibition of mold.

Depending on species, generally 70-75 F is optimal temp.
Relative humidity = 48-57