I don't ride with a group, so paceline riding etiquette is usually a moot point. However, I do ride in centuries. Lots of times I'll look in the mirror and see someone right on my tail. I'm 6'4" so I throw a fairly large wind shadow, so I guess I attract tailgaters.
Other times someone will pass me, but then not pull away; they'll stay a few feet in front of me. In those situations I'll often tuck in behind them for awhile.
Is there any etiquette about drafting someone during a century? I've had people ask before - "Do you mind if I tuck in behind you for awhile?" - but not often.
Personally, I don't mind if someone gets behind me. I'm going to be riding anyway, and if we touch wheels he'll go down, not me. But I don't want to make someone mad by tucking in behind them if it's against "the rules."