I know this is an old thread, and there multiple ones on the Kohler Villager 5' cast iron tub, but I figure this is the one most germane to share my experience leveling it....
I just received my 5' cast iron Kohler Villager tub this week, managed to get it upstairs with an appliance dolly with the help of my son, and fit into the alcove using the technique Terry described in another thread (thanks!).
Because I have some access to the sides of the tub, I decide to follow the manufacture's recommendations and level by shimming under the feet.
Before I got the tub, I prepped the floor by placing 5x8 BOCA plates on the floor where I knew the feet would be. I very very carefully made sure they were all level (using multiple plates, and even cutting out my own shims in the same dimensions) and screwed them into place. I was hoping this would make for an easily leveling job....
I can attest that the feet on my tub were definitely NOT machined level... I had accurately leveled pads (using my new Stabila level) and the tub rocked significantly.
The tub is tough to level. The way the feet are located in relation to the center of gravity of the tub, it's very easy to shim the tub and not realize one of the back feet is now off the ground. I also found that the apron on one side would contact the floor so I had to add more shimming to all four feet to raise the tub a little higher to prevent the apron from taking any weight (as per the manufacturer's instructions).
My real nightmare was it took me a while to figure out my tub is "twisted"... maybe normal for cast iron tubs? What I mean is, if I got the long back edge level and then got the left side in the alcove level, then the edge on the right side of the alcove was so out of level that there was over a 1/4" difference from one side to the other. I spent a lot of time on the phone with Kohler arguing about this (I'm not a pro, so maybe this is expected and maybe I'm out of line here). So I ended up "splitting the difference" so the right edge slopes into the alcove (a little over 1/8" over the 30" span) and the left edge slopes out of the alcove the same amount. I guess I'll have to carefully cut the tile to match the slope, but at least the long back edge is level.
After all that, if I had to do it again, I think I would just put down a thick pile of plaster or cement, push it into place until it was level, then let it set-up. Maybe a combination of the ledger Terry suggests with the plaster/cement, this way the ledger just acts as a leveling guide but the plaster/cement takes most of the weight of the tub.
Sorry about the long post, but hope this helps others.