People are often leary about tightening up the tank. A little movement is okay, but you can get it almost tight to the bowl if you do it slowly and evenly. You don't really want tight porcelain to porcelain, almost touching.
As jadnashua said, some people are really unsure on how tight the porcelain should be. I personally like to couple them as close as I can so they don't wobble. If your scared of tightening any more, put a plastic wegde where it won't be seen to stop it from moving.