I gotta tell ya, I bought casters for a cab I was planning to build (two swivel, 2 non..) and a week later went to an auction, picked up a space harrier cab for $25 that had leg levelers on it..(the high-quality swivel ones, more round on the sides (sorta shaped like cup-shaped carpet protectors you'd buy for a sofa), not the flat bottom cheap ones like on a fridge..and i thought...hmm..if i'm only moving it a short distance, these slide pretty good..and they did! so in the end I took the two non-swivel castors (2" wheels) and mounted them on the back base of the cab, about 3/4" up from the bottom edge of the plywood (with carriage bolts).. now if i tip the cab back past 33 degrees, the wheels touch the floor and I can roll it around with ease, set it where I want it and slide it the few feet i have to to get it in place..
don't have to worry at all about it sliding during gameplay (although most people will tell you that once the castors settle into the carpet it's not goin anywhere anyways..:-)
I think its a personal choice...either way works, just thought I'd throw out another idea...