It sounds like you are using both grade 4/5 and grade5/6 books. If you are teaching fifth grade and your kids have had Investigations before I would recommend the following:
Math. Thinking at Grade 5 (this is not the same type of intro. book as the Math. Thinking books at grades 3 and 4)
Picturing Polygons (2-D geometry)
Building on Numbers You Know
Name That Portion
The above units are all essential (in my humble opinion)
What you do after those really depends on what the kids have already done. Maybe your district should make a long range plan so that the major topics get covered over a 3 year span. For example, if you do Changes over Time and Seeing Solids and Silhouettes in 4th you may not want to do Patterns of Change and Containers and Cubes in 5th. I highly recommend Between Never and Always since that probability stuff doesn't show up any place else. Measurement Benchmarks is also a good one to include since there isn't much measurement in 4th and there's no volumetric measurement in the measurement unit in 3rd.
