As early as possible, and well before the joinery is due on site. Early involvement lets the workshop flag issues like misplaced power points or bulkhead clashes while they are still simple to fix.
Cabinetry usually fails to fit because tolerances were assumed rather than built in on purpose. Sites are rarely perfectly level or perfectly square, so joinery needs a few millimetres of built in adjustment.
A shop drawing catches clashes between joinery, services and structure before material is cut. Fixing a problem on a drawing takes minutes. Fixing it after the cabinet is built can take days.
A good joinery partnership is built on five things: proper shop drawings, agreed site tolerances, early site sequencing, respect for design intent, and reliable follow up when problems come up.

