I have a theatre-style popcorn machine that lets the steam escape (ever so slightly) as the pot is overflowing. When I want to make a smaller batch, I do it the old-fashioned way: in a fully covered saucepan on the stove. When both are preheated properly, oiled and salted properly, it's hard to tell the difference. I do know this: a tightly covered pot will heat up more quickly (because it's retaining more heat), resulting in the kernels popping a little faster. That's simple science.
From the time the popcorn starts popping until it's removed from the heat, it's only a very short time. The average popped kernel is probably in that environment for a minute or less. Whatever steam the popcorn absorbs in the covered pan it quickly lets off when poured into the open bowl.
There might be a minor difference, but if you have to ask, you are probably better at making nearly perfect popcorn than anyone you're likely to ask. Getting the heat, oil, salt, and other flavorings right along with having fresh kernels of a variety that pops to your liking are going to have far more effect on the finished product than minor venting of heat and steam during the popping process. Once you have perfected the rest of the process, you'll be able to tell for yourself whether to cover tightly or not has much effect.
The great thing about figuring it out for yourself is all the popcorn you get to eat. :-)