"He who marches out of step hears another drum." ~ Ken Kesey
To me, if te art is solid as in no halftone dots, and you are printing white over white, you have no reason to choke one or the other. Your concern should be to have it registered dead in. Inkmann has a good method. It's also what Pierre uses. The white is intended to be 100% white (all over).So why choke it? You normally only choke a color so the it does not peak out the side and show it's color. In this case, white is white. If it's off a little, it won't ve noticed.The only reason I can see choking I guess is to help maintain a clean edge. If that is the goal, then you would choke the top color for sure.Imagine how they build peramids. They get smaller as they get higher. If they did the opposite of that, the 2nd layer would collapse over the first and you would see a stair step down. That would give you a funky edge. So if you feel the need to choke, choke the top color.