I regularly delete my forks once I'm done with my pull requests. I used to do keep-ups (fetch from upstream, merge into my own, push into my own)... then it got tiring and out of hand.
And I haven't even contributed much to open source (very very very few pull requests)! So I started deleting repos once I have not contributed for > 6 months
A great idea, but I personally think it is good to have these contributions visible to anyone who is viewing my profile- demonstrating that I'm someone whom one would like to work with.
The contributions would still be visible (under contributions, and repositories contributed to), it just hides the fork (and simply having a fork doesn't mean you have contributed anyway).