I agree that the primary goal is to ensure the outfeed surfaces are in the same plane as the table saw top. Doesn't matter if it is the same plane as the floor or level to the world. This is the principle used in leveling machine tools.
I'm still working on a woodworking workbench for my shop, but the supporting frame is complete. I used six 4x4s as the legs and put 1/2" dia. leveling feet under each one (legs elevated by flip down casters in the photo). I plan on leveling it so the work surface is flat, whether that is level to my shop floor (modern slab with very slight drainage angle) or to the world. I've used these same type of leveling feet when leveling metal lathes. On these legs I put a fender washer between the adjuster nut and the bottom of the 4x4.
"Quality is like buying oats. If you want nice, clean, fresh oats, you must pay a fair price. However, if you can be satisfied with oats that have already been through the horse, that comes a bit cheaper."