For comforts sake when ferment my cider (which I always do in 5 gallon carboys), I just put in 4.5 or 4.75 gallons plus yeast. In my house the extra apple juice will get drunk by the kids and it's just a more comfortable fit. Since I keg my cider, it doesn't really matter to me if the keg is slightly less than completely full.
Currently On Draft: Bamberger Rauch Dunkel, Belgian Blond, Pilsener Urquell clone, Smoked Porter
Bottled: Concord Pyment, Mi'Apa Sparkling Mead, Chimay Blue, Old Simcoe American Barleywine, Old Cantankerous
Fermenting and Conditioning: Pseudo-Decoction Munich Dunkel, Left Hook Bitter
Recently Kicked Kegs: Fresh Hop Pale Ale, Citra Rye IPA
On Deck: Old Rasputin, Northstar IPA, Ur-bock Dunkel