My opinion is that finding the spine allows you to ensure the rod is allowed to bend the direction it naturally wants to bend. Doing this will keep teh rod from fight ing itself or you when you try to direct the line to go a certain direction.

I find the spine on each section, then assemble the rod and double check how it comes out. If things are not lined up properly, the heavier sections will over ride the lighter ones. I check my findings until they all agree. which they will.