Bajorans who had been attainted were not permitted to wear their earring.
Ro Laren wore a Bajoran earring to symbolise her pride in being Bajoran. However she did not follow the Bajoran faith so wore the earring on the wrong ear to discourage vedeks trying to feel her pagh, which was normally done by squeezing the left ear. (DS9 novel: Avatar)
In 2369 Ro's earring was useful in locating her when she was involved in a shuttlecraft accident. The unique metals used in it's construction allowed it be more detectable in sensor scans. (TNG comic: "Light of the Day")