To give a different perspective. I always forgot to check the box, and now noticed a significant decrease in randomly left over branches. So it's working out pretty well for me.
It seems like it would be possible to have the best of both worlds by defaulting to checked for first PR from a branch, but defaulting to unchecked for subsequent PRs from the same branch (because if you didn't delete it last time, you probably don't want to delete it this time).
