Don't know about shimming the closet flange but I did recently learn a valuable plumber's trick with the hold down bolts. I had always had trouble lining up the bolts and getting them to stay put while installing the toilet etc. Then they may or may not be in the slot in the closet flange when you are ready to put the nuts on the bolts.
The trick is to use two additional nuts to secure the bolts to the closet flange before you install the toilet. Just put the bolts in the slots and then the extra nuts and tighten them to the flange. Then sit the toilet on the bolts and put the hold down nuts on. Lots easier!!!!!