There is no need to add, but we love to mix things up. We work on math at least 1 hr. per day - we use MM4 and I pull 16 pages out from the different chapters of the worktext per week (now working on Multiplication, Measurement, Angles, Geometry) - most weeks we get them all done. Also, one time per week we do 2-3 pages from flip math, pattern blocks, color tiles, c-rods, geoboard,logic safari, weeklly Reader Tables/Graphs, and mental math books along with either computer recommended games from MM or board games. Ds does not like flash cards so I have him recite his facts, skip count, etc. at least 2x per week. I keep a master list for daily reference to date and check so I do not forget anything.
MM does have suggestions and the web site has information - I have e-mailed with questions and received a timely response. So the support is great.
I highly recommend buying it if your child works well with a mastery based program. And only add if you feel the need.