Just shim it then caulk. If you do decide to use grout, as long as there's enough depth by the shim, grout over it. Note, depending on the grout you use and the tile, you may never get the toilet up without breaking something...polyseamseal works well, I've read. Instead of buying shim material...coins work well...just don't use a steel penny! None of the current production US coins will rust, and that gives you a variety of thicknesses with no special trip to the store.