Definitely leave some head space or you'll have a mess on your hands (and floor, and walls, and ceiling... armchair covers, throw pillows, and carpeting...). How much head space you need will vary on the yeast and the beer, but most people do 5 gallon batches in them.
My Bar Build
Primary: Sweet stout w/ pink peppercorns, Kriek, American Brown
Secondary: American Barleywine
On Tap: Pumpkin Spice Ale, Coffee Blonde, Belma APA
Bottled: PB&J Sweet Stout, Rhubarb Berliner Weisse