I've been making wine for two years now and I use 6.5 gallon carboys.
I've heard a lot about people topping up to the top so that there isn't
any oxygen to oxygenate the wine, however, I've found that once
fermentation begins it pushes out the oxygen and leaves co2 in the
carboy. I have had no problems with this method. The only concern
would be when I rack in to another carboy, but at the same time,
generally, you're supposed to stir the wine in order to get the co2 out
of it. Like I said, I've had no problems with this but I was just
wondering if anyone has any advice.