I use a Badger 350 so my cleaning routine may not apply to all.
If I am painting similar colors, such as military flats, I simply flush between sessions but enough residue remains behind that soon the performance does degrade and a break-down is needed.
I always breakdown and clean 100% when:
Switching between different types of paints such as acrylics, enamels and lacquers.
Painting anything with yellows, reds, greens or blues.
Painting metallics of any kind. (Those little metallic flakes don't look good on that white or clear finish-believe me, I know!)
Before spraying any clear coat.
Always after any kind of acrylic.
Just my rules...