Charge move breakpoints do exist but there's two reasons why they aren't as important as fast move breakpoints. Firstly, since you aren't constantly firing them off compared to your fast move their contribution to your overall DPS is much smaller than hitting fast move breakpoints.
The second reason is that in a general sense, charge move damage increases pretty linearly with power-ups. In most cases, as long as you keep powering things up they'll continue to do more damage. Since you likely aren't firing a constant barrage of them however this additional contribution can be rather small. This fact compounded with the extremely high power up costs at upper levels calls into question the worth of maxing out *everything* versus only hitting the last fast move breakpoint. This depends on how you play and how much dust/candy you want to drop into something.
tl;dr: Charge moves get stronger if you keep powering up, but their effect is much less noticeable for a significantly higher cost.
