FWIW, I was getting ready to start putting the plumbing in place and got uneasy about the shims under the tank trim. I ended up getting a 1/2" piece of hardwood ply and shimmed under it where necessary. Then used putty to fill the visible gaps to make it look nice, painted it up and now it's ready for the tank. Having done this I feel MUCH better. I was about to use the self leveling method, but it was going to add more work/cost to the situation and I was worried about my execution. In the end, it's moot, I think my problem has been solved.