If you've ever welded on both sides of one of those butt clamps, you see that the panel shrinks/pulls together such that the clamp can be difficult to remove. This is an indicator that the panel will be losing crown in the case of a crowned panel, ie: roof skin, hood, etc.
Part of the reason we use butt welds is to duplicate the original metal "flavor and feel" so that when someone looks at the backside of a panel or under the floor pans, they don't see a flange repair, or have to contend with ghost lines on exterior painted panels. We can effectively cut out damaged (rusty) metal, butt weld in a new section, planish/metal finish/filler as needed, and we can have an invisible repair that for all practical purposes should be a permanent repair. Part of the reason you see recommendation to use TIGHT butt welds, is that it eliminates open area that allows any panel movement, ie: as the weld shrinks, the panel pulls together. If the crown of our patch used to match prior to welding, weld shrinking along with the panel's pulling closer together (due to a gap) is going to pull in some of the crown. If it is a flatter crown, such as the middle of a quarter panel or top of a roof/hood, it is more noticeable as the crown in these areas is normally minimal/just enough to maintain the shape, and may result in oil canning. If it is a concave crown, such as a reverse where the wheel opening flare comes outward, then the weld seam has a tendency to pull outward as it shrinks. The following pictorial shows an exaggerated crown and gap primarily so you can see it in such a small area. But it shows what happens as weld shrinkage and panel movement pull at the surrounding area. In the bottom view, the red line depicts where the crown was originally.
With tight butt welds we do need to planish in order to remove any deformation caused by shrinking, and add some stretch back into the area. This should be the extent of our planishing effort, as the tightly fitting panels prevent the panel pull from adjacent areas (that a gap allows, resulting in loss of crown). In the case of any gappage around a patch, we would need to planish even more to add enough stretch to overcome this loss of crown, or add filler as needed.
In panels like hoods, trunk lids, and roof skins, that continual crown shape is what helps support the entire panel free from oil cans. Any low spot derived from a gap (panel pulls together) combined with weld shrinkage may serve as an interruption to that support.
Looking at your quarter panel, one would expect in the case of a normal butt weld that the edges of the panels against each other will be a positive stop against any pull from adjacent areas. The only planishing we should need to do is to overcome any shrinking issues due to weld heat. Absent any planishing one would expect the panel to show the following signs:
If your welds show any of those signs, then additional planishing is in order. The lower part of your quarter does appear to have some low areas that should need more planishing to bring them up.