yes and no, yes there are "breakpoints" for charge moves but no they don't really change much.
E.G., a maxed ttar vs gengar with crunch does
98 damage with 13 attack
99 damage with 15 attack
vs Alakazam
80 damage with 13 attack
81 damage with 15 attack
VS Mewtwo they are the same (79 damage)
So yes there is a difference, but when you consider that you'll likely get 4-8 crunches in per raid per Ttar, that comes out to ~1 Bite damage difference or 0.5 seconds.