Before that. It can be helpful in getting to, and/or verifying, the 'built right' stage.
Following that, lets say on a full recurve, when I lay my limb pattern on the limb to trace it, I have a small hole drilled in the pattern at the deepest part of the curve, and align it so that I can see the centerline through the hole when I push the pattern down into the curve. Otherwise, even if the pattern is centered at the handle and tip, it can move a little to one side or the other as it is pressed down against the blank, which can ultimately cause misalignment and limb twist.