Don’t worry about how level your floor is.
Lock the casters and, if close enough to level, use a tapered shim to make it solid. If you don’t want to make a shim, buy a pack of plastic (moisture proof) door and window fitting shims at the home center.
If not level enough, use tapered shims under the low wheels to level and make solid.
If it is far enough out of level that you can’t do the above, either use leveling feet instead of the casters (this can take a little work, since the caster base plate is intended to distribute the load) or use 4X4’s under each side with leveling feet or shims (a little higher and takes care of load distribution). Steel column base-plate shims, if needed, protect the floor and allow rough leveling. (Column shims are the steel plates used at the base of steel columns, and are available from steel yards and some building supply yards, in sizes from maybe 4X4 inches to 12X12 inches or more, in thicknesses from 3/32" to 1/4 or 3/8", and at least to 1" thick in larger sizes. They are handy as inexpensive, precut steel in plate thicknesses)
My lathe and my metal shaper are both on shim stacks, as well as my workbench, and several of my drawer cases. The lighter things are on screw levelers (the drill press is about 400Kg, so it is on 18mm screws with 1/4" (6mm) 100mm square plates under each)