Oh I remember asking this question before Harry was born, cos DH and I were wondering about it!
I found out that if it's not totally retracting by itself, you shouldn't pull it back cos that can actually damage the little joiny part that is still joined when they're little. So to just leave it to do it's thing.
ETA: Found the thread in question and bumped it up for ya. It was way back in September!! It's in the baby section "those of you with boys"
Edited by arohanui