When and how do you create a whole-robot design?

[ohrly?]

What I mean by this is, when do you transition from making mechanisms to putting those mechanisms together into a robot?

Traditionally, our team has struggled because we have built mechanisms, and then we put together a really bad combination of items that don't complement each other. Things like using mecanum wheels that don't actually have the traction to put out all the force our gearboxes can provide (we gear them too high, so we go slowly without any added push), especially when strafing offered us no strategic benefit. I'm frustrated by it, and I can't say I'm not part of the problem, but my team HAS to change the way we design our robot.

Any advice?