To shim or repair uneven tile?

I've got one tile that rises as it approaches the flange under the bowl. Of course this causes the bowl to rock and is a single pressure point under the bowl since it sits on that high corner point. Should I just shim the bowl or should I try to grind down the offending tile since it's under the bowl?

I'd pull the tile up and fix the problem that is causing it to be uneven. There are adhesives that you can use to stick the tile back down. In my opinion, shimming the toilet would not be the best way to go because it is covering a fixable defect. Grinding the tile down might work, but again I think fixing the cause of the problem would be better than fixing the result of the problem.

If you're saying the bowl rocks on the flange without a wax ring in place, you've got a real problem. If it's just rocking on the floor, you could get away with shimming, but I agree with Gary -- better to fix it right while it's relatively convenient. Can you post a picture of it?

I'd probably shim the toilet and leave it alone. It would be better, but since it's covered up, not much, to replace the tile. To do that, remove the grout around it so you don't damage adjacent ones, smack it good in the middle (wear eye protection) to break it, then dig out the pieces working from the center out. You really want to use thinset rather than mastic which may require buying a big bag when you'll only be using maybe 1% of it (the stuff won't stay good for all that long once opened).

I've never met a perfectly level tile floor yet! Use plastic shims or pennies to shim the toilet and caulk it with latex caulk such as Poylseamseal or, Phenoseal.