There is also some out here who feel the stranded water proof fillers help with minimizing ghosting or mapping over welds compared to traditional fillers, so I figure thats just one more insurance I can add to the equation.
I've heard that mentioned as well, and one of these proponents in particular also expressed to me the need for strengthening the weld seam. In retrospect, I'll add that most mud daubers put his work to shame, and his weld seams needed all the help they could get to keep from falling apart, as weld penetration was non-existent. So considering that source, this need/recommendation that he was readily handing out in his use of fiberglass over welds was nothing more than a Band-Aid to sub-standard work. Not to suggest that you are putting out that type of work, I've just heard quite a bit of the same recommendations that no doubt came from similar sources and have been repeated enough where they are now accepted by some as a requirement over welds. So me being outspoken (don't know when to shut up), I'll normally take the bait and put in my $.02.
In the context of lapped seams, I think we all can agree the ghosting is a result of differing expansion rates of 1 vs. 2 thicknesses of metal. With that in mind, I would suggest that excessive weld proud left on either side of the panel can produce a material thickness that far exceeds just the two panel thicknesses of a lapped seam, where any excessive weld left behind may also contribute to a ghosting effect. IMO the back side should be dressed just like the front, if access is available. In the end we should have a panel with consistent thickness throughout, including the weld, to lessen any chance of ghosting (to the best of our ability).
Looking at this picture of a 1951 triumph Saloon, the forming capabilities of the time (limited as they were) meant the rear window area/roof attachment required some welding.
Looking at this area from the inside, you can see the seams that were done at the factory. There is a noted absence of fiberglass impregnated filler, but more importantly, there were no ghost lines to be found, anywhere.