This was moved into here to avoid sidetracking another thread in the Traditional Catholicism forum.
I see everyone’s point that has commented about this. However, I guess I did not make a complete statement. Must and should are completely interchangeable except in grave circumstances. MarkThompson is correct in what he said about this.
Basically, the issue that occurred with another person on the Parish Liturgy Committee (the DRE) is that we got into an argument over the use of “must” and “should” as used in the GIRM. I say if you should do something, you should do it! However her view was that everything that we “should” do is 100% OPTIONAL all the time! That was the real issue. Also, I have been slowly implementing things at my parish so that we are even doing the things that we “must” do/have. I have yet to be to a Mass where nothing is illict.