Do waterproof boots need additional waterproofing?

After a certain period of time (depending on what type of waterproofing substance is being used) yes if the waterproofing is on the outside. If you have a lining on the interior of the shoe or boot (Gore-tex does this a lot) then you will very, very rarely ever have to refresh this waterproofing layer. In that instance, the outside of the shoe will become wet, but the interior will stay dry.