I would not ferment in a sealed container. If it's a mason jar with a lid, after I add new milk, I tighten the lid, shake a little (to get any persistent curds from the last batch dislodged), and then back off the lid so that the co2 can get out.
I don't think it's a big issue, but I read that keeping the grains under constant pressure can cause them not to grow the right way. Apparently they get small and dense instead of large and lobular.
If you prefer a really fizzy version, after straining out the grains, let it ferment a little longer in an airtight container -- in the fridge if you want it sour (the acetic acid bacteria take control at lower temperatures), on the counter if not. I think. It's been a while since I've actually fermented kefir on a regular basis. (At this point, I let a pint of it ferment in my fridge for a week, then I use that for biscuits, or pancakes, or some such, in place of buttermilk.)
