Centering rings are just that. They center the wheel on the hub just as the original hub centric wheel would. This is good because it helps with any vibrations there might have been without them. The hubs don’t really hold any vehicle weight but might help the lugs with impact forces. All of your clamping forces come from friction between the wheel and mounting surface, properly torqued on the lugs.
I have run them before and while I prefer hubcentric I have never had any problems getting them centered. Basically if you don’t use centering rings you will have to tighten it by hand slowly and evenly. Also, new wheels will be pretty easy to center. My advice is to just use the rings. Most wheel places will include them with the kits if you ask.