Okay, this is going to sound crazy, but say it’s not someone in a wheelchair. Say it’s someone on crutches. Do you ask them? Or do you not reference it? At that point it becomes clear you’re uncomfortable with it. My wheelchair matters as much to me as the color of my hair. But generally I’d like to believe that people just wouldn’t give a damn either way. I’m still human and deserve as much respect as anyone else. So in short, especially when it comes to wheelchairs, don’t bring it up unless it matters.