Why can't you wear nail polish when you get gas at the dentist?

I have never heard of this, but I remember someone once telling me that when they were Cheerleading, they could not wear nail polish. The reason was because if one of the girls became injured, once can easily ensure that the heart is still beating by pressing the nailbed and watching the color fade and return. Similarly, if the nailbeds go white, it indicates poor circulation--you cannot pick up on this indicator if the person is wearing polish. I do not know if this is accurate, or if it is the same reason why this might happen at the dentist, but it is a guess.