What would be the best configuration for casters for a workbench?
The best set-up for maneuverability plus stability to to allow the workbench to sit on fixed legs.  When you want to move the bench you can implement a jacking system with the casters similar to 



I have seen this implemented with a rope where you can pull the casters down underneath to lift the bench or with a foot operated jack similar to some table saws such as 



Hope this helps you out.  I've tried the casters with 2 locking casters and I've never been happy with the stability of the bench.