to undercoat or not to undercoat???

I would agree with the last few guys, I wouldn't undercoat. The areas where the rust gets its start is in the box-in panels ( cross members, frame rails etc.) and most specifically between the pinch welds that hold the different panels together. My suggestion would to be to use a product called WaxOyl that is applied with special air pressured gun (runs around $120.00). It's exactly what it sounds like a waxy oily sunstance that is fogged into these areas with the gun that comes with some tubes with different tips for getting into these areas. Once fogged in the product creeps for the next few hours into these impossible to otherwise protect welded areas. After its done it's moving it drys into a waxy film that you can wipe off with some degreaser the nicely painted main areas of your floor pan that you like to admire.