I've seen cars as late as the early-'60s without even a driver's side mirror. I don't know when these became standard and/or required. My preference is to use the car as it came from the factory. If it didn't come with the passenger mirror, then I don't use one. It takes a little more effort, but as long as you're not changing lanes or backing up without knowing what's there, there is no safety issue. Truth be told, I think it's kind of cool to have no passenger side mirror, some people are surprised to find that they weren't always there. Same with the shoulder belt.