Jim,
As others, including myself, have mentioned the switch could be installed after skinning the bottom first ... but in my opinion you are making the instillation harder on yourself.
If you have a young back and want to lean over the wing, go for it. If your knees are in great shape, then working through the inspection plate hole while the leading edge of the wing is hanging over the edge of the workbench is doable, but is not a piece of cake. Being able to slide the skin on and off to get the washer buildup and vane position correct is truly much easier on the body ? so that is the path I took.
My suggestion ?. before installing the inboard or outboard skins, place the W-1202 skin in position to get a feel for the level of difficulty of accessing the switch assembly with tools through the inspection hole and base your assembly decision accordingly.
John