Head Room

Instead of 6.5 gallon carboys or buckets, I'll be using standard unused 5 gallon paint buckets for fermenting (I can get them for free versus the 14 bucks a pop I would be shelling out for the 6.5 gallon buckets at the brew shop). I have two 6.5 gallon buckets, but they will be in use for beer while I'm fermenting my cider. I know beer needs plenty of head room while fermenting to prevent a foam over mess, but does cider need the same amount? I was hoping to fit 4.5 gallons of cider in a 5 gallon bucket. Will half a gallon of head space be enough? Or should I only put 4 gallons in and leave a full gallon of space between the cider and lid? OR, should I just stop being a cheap skate and just shell out the 15 bucks to do it right? The lids for the paint buckets will be air tight, with a rubber O-ring to boot.

EDIT: The only extra ingredients will be a half pound of white sugar.