Right, so in designing a solution, you have to design for the worst case. Since you can't always be assured the luxury of looking at your hands, you have to learn to be proficient without doing so. IMHO, anyway.
Good point, but I would consider this an argument for frets, not lines, because even with lines, it's still pretty easy to "sound bad" if you aren't exactly on, and only the ability to hear yourself can fix that. If you get dirty looks from the singer, how will you know if it's a "sharp" or "flat" look?
If you think there's a potential you won't be able to hear yourself, then I'd suggest you always have a "plan B" for those cases, like an IEM system of some kind. They're very affordable, portable, and phenomenally effective.