The theory is that a horizontally growing branch has it's top side in constant tension due to the weight of the branch hanging down and thus it will make a good bow as the upper sapwood surface has been used to tension all it's life.
Conversely the sapwood on the underside of the limb has been in compression all it's life due to the weight of the branch and theoretically might not be so good in tension.
I tend to just look at the growth rings myself as I've found hugely assymetric rings in vertically growing limbs as well as horizontally. Add in the fact that the underside of a branch is far more likely to be clear of small branches and hence knots, and your choice isn't so straight forward anymore!
So I take the view of listen to all the advice, then do what you want and re-appraise the results afterwards.
If your findings agree with the received wisdom them take it on board, but if your results are good then go your own sweet way.
With natural materials it's difficult to make hard and fast rules. Maybe the underside of a branch of 'good' Yew could be better than the top side of a poor piece.
It's like all the 'armchair experts' I've hear telling me "You can't make a longbow from English Yew"... I've had some beautiful English Yew, I've also had some not so good, but it's all made shootable bows.
So that's cleared that up
Del