I wonder how much not being perpendicular to the string contributes to the dreaded bow bounce?   Looking into a mirror its kind of hard to tell if its perfect.   This is another instance where it seems an advanced technique, double stops, is useful in early development to obtain a true unwavering bow stroke.  When you bow two open strings at once, you get an instant audible feedback if you are bowing too strongly into one string or another. 

The added pressure necessary for two strings is good for the muscles in the forearm and elsewhere which speeds up the "getting the right arm in shape" portion of development.