Papazian had an interesting theory on carbonation and headspace in his book. Off the cuff, I can't remember what the whole dealio was but he was mentioning that if you leave too much headspace there is more 'room' for overcarbonation to take place. Those growlers (at least the ones I have) are fairly hefty glass. I put a primed cider in it and it was fine. But to be on the safe side, I'd still follow fiddup's advice. If you are shooting for a fairly high amount of carbonation that is a good number for six gallons, assuming you are pretty much fully attenuated on your wort.