Often the floor is not perfectly flat, so the first thing to do is set the toilet in place with no wax and make sure it does not rock. If it rocks you need to shim it so that it cannot rock.
Every toilet is a little different on the bottom, you probably do not need thicker ring if it is flush with the floor.
Attach the bolts to the flange with a 2nd set of nuts; this will not only hold the bolts in place but also ensure that you set the toilet centered over the flange. Most screw-ups happen when people set the toilet off-center on the wax and then push it over to get the bolts lined up.
I have always set the wax on the ring first. Gravity works that way.