How to Arrange Casters
When most people put a cart or cabinet on wheels, they buy four swiveling casters and put one at each corner (Option A). Two other configurations are also worth considering.
You could substitute two fixed casters for two that swivel (Option B). Alternatively, you could move these casters to the center of each side of your unit (Option C).
What's with all these options? Well, the choice comes down to whether you want your unit to be easy to maneuver into a tight space or easy to push in a straight line—or somewhere in between.
Option A, with swiveling wheels at all four corners, is best for parking your unit in a tight spot. If your shop is small and crowded, this is the way to go.
Option B, with fixed wheels at one end and swiveling at the other, is arranged just like your car or a grocery cart. A unit with this configuration tracks nicely and is easy to steer; it's best for shops with plenty of room.
Option C, with wheels placed on centers rather than in the corners, is a good compromise between A and B. Carts used in railway stations and factories years ago were usually built this way. They could spin on a dime, but they also tracked well. This configuration does have a couple of disadvantages, however. First, the corners are tippy, so you must evenly distribute a load. Second, this arrangement doesn't work on a cart with four legs. It's really meant for a cart with a platform base.