Quote Originally Posted by AlexanderSextus View Post
so when would they use the quincunx instead of regular battle lines (and vice versa)?
Your question sound as if within the boundaries of the theory stating that the quincunx formation was held even after the armies had clashed; the mainstream view on the quincunx though is that the three battle-lines was joined ( probably with small intervals between the units ) before the battle, and that the maniples were only for temporary maneuvering.
So the quincunx and regular battle lines are actually part of the same system, the triplex acies quincunx, although they have been used both together and apart.

Regarding standardization of tactics, none such existed in pre-marian times either, although some, like the abovementioned system, was commonly used.
Battle-reports, the few we have in such detail, indeed suggests that manipular/manipulatory tactics were used less frequently in the late republic but also that the romans still favoured multiple lines of engagement.