I dont think that could really be challenged as the use of "should" "may" "shall" etc is all throughout the CFRs and has always had the same definitions - I do know of some old legal arguments years ago that blurred the use of the words, but in my "previous life" the use of these words was very strict and had very specific meanings. The writers of the CFRs have general guidelines to follow and some of them include the use of these words. "Shall" means a regulatory requirement, "may" means its allowable but not required, "should" is usually interpreted as "you better have a damn good reason for not doing it"
Hope that helps
