From all that I have read, to get the best percentage, is to upgrade one ring as high as possible (one person mentions +15 being the best possible for drops) and don't bother upgrading the other, the second ring gives a bonus, but no additional by upgrading it.
I have only had a couple games with two rings of wealth, so I haven't done much testing.
Best strat is to get a +4 from the imp, reforge in another 2 rings of wealth and apply 15 scrolls of upgrade. Then have another +1 for a total of a +23 on your ring of wealth. With other rings you can get up to +24 if you have a +2 instead. You can boost this up with the rouge talent that adds +3 levels on your ring for a total of +26 levels on your ring of wealth, +29 on any other ring.