That is true - the seventh cervical vertebra IS a much more reliable guide. The vast majority of horses (even so-called uphill horses) will be slightly downhill, a few will be level, and a very small few will be truly uphill.
My point in mentioning the racing thing is that all racehorses are bred for the same thing - SPEED. So, they will all have a lot of characteristics in common. What do a racing QH, a racing TB, and a racing Teke all have in common? In my experience the majority of ALL 3 breeds are markedly downhill. It's less noticed in TB's and Tekes because they have higher withers than QH.