oric_dan:
...and they certainly work best on surfaces where the wheels can, in fact, "skid" easily (eg, not carpets).
Well - they'd work ok on carpet - the carpet just might not survive all that well...
oric_dan:
If outdoors on earthen terrain, I don't think having casters is a good idea at all, as they will probably dig into the dirt a lot.
Depending on the size of the robot, you could go with a couple of larger "off road" casters; they make these larger casters that have knobby tires for such purposes. Not ideal, but they do exist...