Raven, my female dobie was really abused before I got her, and had real issues with men. She was not mean, but fear agressive. I did use a muzzle on her at the vets' office for his hands on exam until I was sure that she would not bite him. She has not had to be muzzled in 7 years. I never tried muzzling her off leash. If I was that concerned for public safety I would not let her loose. I would be afraid that the muzzle would give the impression of a dangerous animal and if something DID happen, it could be an admission of liability.
Stella