Well, I understand the theory, but I'm not sure I'm willing to buy that. The krausen falls back into the beer and ends up as trub anyway, even if it doesn't blow off. I think that I'd go with the 6.5 gallon to have the headspace, and not risk losing a gallon of beer by not having the headspace.
Broken Leg Brewery
Giving beer a leg to stand on since 2006