Markos, it is true that the 1979 book says "may" or "on occasion" at nearly every possible opportunity. About the only things that are absolutely fixed in the eucharistic liturgy are the reading of the gospel, saying the creed on Sundays, and the anamnesis (if you aren't in Rite III you are stuck with a specific eucharistic prayer text as well). The way nearly everyone interpreted this for the first twenty years was to ignore all the options and do exactly what the book says, no "mays" or "on occasions", except most places have some eccentricity which they doggedly maintain regardless of what the book says. One should also note that Anglican books tend to say very little about manual acts (e.g. pre-1979 books only spell out exactly how the elements were to be handled during the consecration-- instructions which, BTW, were in the end widely ignored). There are parallel manuals of usage which tend to correspond to various churchmanships (e.g. Michno is the current high/broad church standard).
The flip side of this is that the general rule has always been "decently and in order". Excepting the occasional clown or jerk, in ECUSA the expectation is that whatever you do, it is to be done with solemnity and seriousness. Mechanical rattling off of the liturgy is frowned upon; if you do the liturgy fast, you really need to try to not make it sound rushed. (There are normal options one can take to speed things up at various points, for instance.) None of this is in the texts, of course, and that's really where the difference has lain. In my opinion, the 1979 texts are better written than the RC equivalents, but the real difference is the manner in which they are used.