Lou noted at https://sourceforge.net/p/tei/bugs/287/#c52c that we commonly use "should" for things that are "highly recommended/mandatory". First of all, I find "highly recommended" and "mandatory" to be quite different things: in my opinion, if you go against a practice, the former would still validate according to a schema, whereas the latter would not.
But more to the point, I think we should fully embrace the use of modal verbs and constructions as in BCP 14: http://tools.ietf.org/html/bcp14
This means searching through the Guidelines for modal verbs and constructions, reading each one in context, and determining whether to fix it. A huge project.
If we accept this, we should add a reference to this RFC document to http://www.tei-c.org/Activities/Council/Working/tcw24.xml .