I put the two fixed ones on the back, and the two swivel ones up front.
That makes the cab a little less stable for side-to-side shaking than doing it the other way, but allows you to navigate REALLY tight spaces with it.
The amount of play in my carpet padding is the major factor in all the play though.
A set of "spike" legs would stabilize it better, but wouldn't allow it to move as easily.
As far as why I did two of each, rather than all four swivelling, it helps guide the cabinet straight when they don't all swivel.
The two non-swivelling ones will force the swivel ones to follow them on a straight back push.
I've got a few tilt back type cabinets, where they have leg levelers all the way around, but you can tilt the cabinet back onto wheels.
That works fine for navigating larger spaces, but won't allow you to turn a corner with the cabinet upright in a small hallway.
I've got a couple cabs that don't have any wheels as well.
Unless your cab is really light (candy cab, etc...) I don't recommend that at all.