Wingnut007 this is the standard way. Check all standard bridge constructions, drainages, frame structure and likes, you’ll see that reinforcements are prepared before shuttering.
Follow my ongoing build XL6 Trillion | Atrium House to see how we do it.
Doing the form before the reinforcement will equally work but in different circumstance. Eg. in location with high water table; the supervising engineer is uncomfortable using the standard approach.
To help you understand this better, there’s no way you’d prepare formwork ahead of reinforcement if you have a t-base design.
Also, installing your starter bars properly is nearly impossible when you do your formwork before reinforcement.