During primary fermentation, your're going to be producing so much Co2, that the headspace isn't a concern. Hell, you could do four gallons or even three in a 6.5 gallno carboy and probably not have an issue.
Headspace may be a concern when using in "secondary fermentation" as the beer isn't likely producing much Co2, so what's in there is all that's protecting agsinst oxidation. Unless you're moving them around a lot, even then it shouldn't be an issue.
At least that's my experience and opinion.
- Brewin' and 'Quein' since last Tuesday.
Bottling the Belgian: A Photo Odyssey
Beer is the mind-killer. Beer is the little death that brings total obliteration. I will face my beer. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see it's path. When the beer has gone there will be nothing. Only I will remain.