I don't do a lot of portrait photography so maybe I'm not the best person to be giving suggestions, but when I have done I have it portrait work I have found the shots much easier to work with in post if the subject was *not* wearing a lot of either pure white, or pure black.
That's my experience. While I do *prefer* the latitude of working with mid-tones without having to worry about blowing out highlights/shadows, it's certainly not a requirement.I did think about those colors, but think my desire for what someone called high key (loads of contrast) works okay with black and white. But it can make the shot a little more difficult with over/underexposure.
I think a lot depends on WHERE you are shooting. I have seen some of the best family shots on the beach is all white clothes. That would be an easier shot than if they were on the beach wearing all black. Also you have to take into account the skin color. A black person wearing all white is very difficult to shoot without a lot of Post Processing.
I do agree about wearing something they are comfortable in because they are just more relaxed in those clothes.