I have read that, generally, Roman men during the Empire would wear only one ring at a time. For instance here: http://m.tribunesandtriumphs.org/roman-clothing/roman-jewelry.htm
An unmarried one would wear a seal, but was there a standard for which hand and which finger to wear it on? It is my impression that when it was about marriage or religion, they went for the fourth finger of the left hand, as they believed the "vena amoris" (a term coined in the 17th century, they were actually thinking of a nerve) was the only vessel that connected fingers to the heart. For example Tiberius, who was after all left-handed according to Suetonius, thus displays a ring in his bronze portrait as the Pontifex Maximus:
I couldn't find sources for how seal rings were worn. I would guess the hand depended on your left- or right-handedness (or maybe shaking hands with the right one had implications), and perhaps the finger was the index or middle one.
Or perhaps there was no standard?