I'd use the airlock. In fact, I use airlocks even in my wines that are bulk aging. Even if fermentation is done, quite a bit of co2 stays in the beer, and things like a small temperature change or a drop in the barometric pressure could cause some of the co2 to come out of solution. It's not a big deal, because the airlock lets it out. In a sealed container, though, it might be a problem.