I think you should leave a gallon or two of head space in your fermenter.
(at least for ales)
I use two 6.5 gallon fermenters to brew 5 gallons of ale.
I think a good amount of headspace is the best policy.
Last brew 9/10/16: Chinook, Centennial, Cascade IPA - 1.066 OG - 51% Pils / 46% 2 row / 3% 22.5L Honey Malt - US-05