Ok, so I gave my first professional fitting today. Sure it was a woman that I am related to, but she let me fit her and ordered some bras.
Her bra (that she was wearing) is a 38B. She is really a 34C. I had her exact size in my sizing kit, in a soft cup style, which she prefers. It was the ideal situation. She got to see how the correct size makes a difference. She even mentioned that nothing moved when she bent over!
So, even those with an average bra size can be wearing the wrong size, and can benefit from a fitting.
If you don't go to me, go somewhere! (somewhere other than vicki's secret, please!) Get fitted!