My MCHN told me to leave it to them and their curiosity and they will eventually stretch it enough. The way my two wrangle theirs I'm surprised they haven't as yet but it can't be too far away!
The only time I've attempted to pull it back a bit was if they had a really runny poo to make sure none was stuck in there, but I'd rather soak them in the bath for a little while to clean it anyway.