As the owner of a dog that is unpredictable around strangers, I should say I don't pet strange dogs but on occasion I do. Not often however. If the owner is there, I always ask though. In fact, it is irresponsible of the owner to leave a dog that may bite and/or has a history of biting unattended. I would never leave Duncan unattended in a public place because of this. Even when I let him out in a friend's yard to pee, I stay with him because I wouldn't want neighborhood kids coming to the fence to pet him and getting hurt.
Regarding the second part of the question, I do not like people petting Duncan without asking me first. He does wear his gentle leader in public and in crowds for more control and I keep him right next to me at all times. That in itself does deter some from petting him, thinking it's a muzzle. But occassionally children will casually run their hands over him as we walk and it's no big deal. I just hope the parents are telling their children after the fact to be more careful.