I would always have my pup off the lead when out in the field, I have always done it, and call it lucky but have never once had a problem regarding the development of the bones.
Since Sash was a pup she was always allowed to roam within a safe range, I suppose if you are confident with your pups training in particular the recall then there is no call for concern. I know accidents do and can happen at times, but regarding the issue with the pup coming across a Charlie, how many people do you know that this has happened too, I know of not one and I have been hunting since late 70's, but I do