I just started a new league this year, and went with 2 keepers. I'm only in one other league, and they just increased it from 5 to 8...its more of a preference really. I try to talk to the guys I have in my league and ask thier opinions on keepers, and gauge it from there. You want owners to be happy in the league they are playing in, so to me, its always "thoughtful" to see how they feel. One thing I would suggest when deciding to make it a keeper league, is to almost start from scratch. Someone's draft philosophy may change knowing its now a keeper league, where as before he didn't have to worry about information like that before.
Best of luck to you!