hm, for me it's kinda hard to visualize what you're describing, as i can't even DO that to my pinky. what i do know, however, is that some people (like gary willis) warn about keeping your pinky straight while playing. as you will already know it is considered good technique to play with your fingertips, using only very little pressure to fret the notes. as i understand it, you're already doing this (your "top" joint is bent), so there shouldn't be any problem. if you feel comfortable using this approach (ie NO pain, enough speed & accuracy), you may leave it as it is.
but keep this in mind:
often our body will let us subconciously develop "non-standard" ways of doing things when hitting certain limits. so the "strange" behaviour of you pinky maybe just a symptom of something else.
also, these links about left-hand-technique may be useful to you:
http://www.adamnitti.com/bass_player_03.shtml http://basslessons.com/basslessons/nitti/technique.html