It's gonna be on a case by case basis (pardon the pun) until John can convince a manufacturer to release cases with sweet spot holes. Pretty much see how your phone acts with the ring.
If you need to be touching the phone directly in order to use the ring then try a skin style stick on cover, if you've got a reasonable amount of distance then you should be right with a non-metal case. It'll be up to you to see if you can test in store (it's very possible, some places are helpful like that) and maybe wow them with the reason why you need to test.
Bear in mind that a normal tag behaves very differently to the rings, you might be best to either hold off for the moment or buy a cheap thin case.