I am grappling with similar issues. I am the "lone teacher" who is using Investigations in the upper grades, and will be sending my students back into a traditional math program when they leave my class. I am needing to assure they are as strong in their use of algorhythms as in mental math strategies. I have barely touched algorhythms this year, and taught straight through "Mathematical Thinking at 5th Grade." I plan on getting into "Name that Portion" after Christmas (I'm currently in "Picturing Polygons"), and feel I need to get into "Building Upon Numbers You Know" to reimforce the multiplication/division concepts that will be important to them in 6th grade.
From experience, when do you focus on the algorhythm instead of clustering, repeated addition, etc.?