Can I delete old back ups
Welcome to the forum Twiggy, I would advise to keep them because they don't take up much space and you never know, that said ccleaner has never caused any registry problems for me and i don't remember reading any on the forum.
I think the best practice is to keep say the last 2 weeks or so of backups for example when the backups are say 2 weeks old and you have no problems then remove them.