Typically I planish before grinding each individual weld dot. Each weld dot will shrink and pull the surrounding panel circumferentially around it as it cools. By planishing as it is sitting there by its lonesome, you can more readily negate these effects.
With any dolly that is used in a hammer-dolly operation, the dolly should closely match the contour of the panel it is held against, without corners touching. So if the panel is flat, the dolly should just off of flat, where the perimeter of the dolly does not touch, only the face in the center. In this fashion, the dolly touches weld proud on the back side, the hammer touches weld proud on the front side. Be sure to hold proper alignment of the hammer and dolly that you don't deflect and distort the panel further. The face of the dolly should be parallel to the panel surface, as should be the hammer face.
So how do you know when enough is enough?
This is something that will vary from one person to the next or job to job based on panel thickness, wire used, technique, hammer force, weld dot size, etc. So my suggestion is to weld a test piece and see how it works for you. I've written this before but I've come to realize the sizes I gave earlier aren't as quick to show the changes, so we'll change it up a bit and see how it works. I'll preface this by saying that this "test subject" serves as a guideline only, and is intended to help you see the effects of shrinking and how the planishing counteracts those effects, more so than establishing a hard and fast measured amount. It is not intended to be the end all-be all of how much, but it should get you in the ballpark of a measured amount to keep the panel in relatively good shape with minimal warping effects during the initial planishing efforts.
For your planishing test subject, you need two sheet metal strips about 3/4" wide by about 12" long. These should be the exact thickness of what you will be using on your car/truck, and will be tacked together on the long edge. They will work best if you have a shear, as you can cut a piece 1-1/2" x 12 and then shear it through the middle for a perfect seam. Thinner pieces cut on the shear will tend to curl so you may need to flatten afterward. For you to see the effects of the shrinking and then the planishing, it works best to be a perfect cut through the middle, so using a shear will help tremendously.
Next, the process and specifically amount of planishing needed is going to be directly related to weld dot size and/or wire type/softness, etc. Now that you have a fresh cut test piece, take your two pieces and align together TIGHTLY along the long sheared cuts and tack the seam at about 1/2" in from one end. (my samples in these pictures are not sized as indicated above, so ignore that part)
DO NOT PLANISH at this point. Go another 1/2" and add another tack. Is there any change in panels positioning? Go another 1/2" and add another tack. What we are trying to do with this process is to monitor how much shrink is occurring. The first tack should "anchor" the two panels together at the end. With the panels tight together, each subsequent weld tack will start to shrink in both pulling the panels together and also shrinking in overall length along the weld. In effect, this will start to manipulate the panels where the individual pieces on the un-welded end try to overlap each other. So If you haven't seen this happen, keep welding dots at 1/2" spacing until it does happen.
Now that you see these panels overlapping, the next phase is to see how much planishing it takes to "undo" the overlap. Start at your anchor tack, and hammer and dolly once.
Go to each subsequent dot and apply the same hammer and dolly in the same approximate force. The flat should be similar in size, but I'd gauge your effort more on hammer force than size of the flats. When you get to the end, check the overlap to see if it still interferes with adjacent panel fitment at the un-tacked end. If it still overlaps, start at the beginning, repeat the planishing of one dot at a time with only one hit per weld, monitoring overlap. When your panel overlap issue has been resolved, your weld dot planishing effort should be the number of hammer strikes per weld dot that it took to resolve the overlap, using approximately same striking force. This assumes your weld dots don't mysteriously grow in size to add the need for more planishing, so again the importance of OCD consistency. This planishing effort will not be the end of the metal bumping to your panel ie: once you get welding in your patch panel/hood scoop/etc. It is the initial needed to help relieve the shrinking effects so the differing forces will relax a bit. What we did in striking one dot once and then move to the next is only for test purposes to identify the number of strikes you needed per weld dot. After this initial planishing, any remaining planishing needed will be based on what the panel looks and feels like, high spots, low spots, etc after welding, initial planishing, weld dot grinding, and panel reading is completed.
Now that you have completed this, just for the heck of it, go to the first anchor dot, and start planishing it and it alone. Keep repeating until you see the adjacent ends start to separate as the weld dot is being stretched. Look at how wide the gap is. This approximates the amount of extra effort needed to overcome the shrinking and panel movement that happens when you leave a gap that size in the panel. It also demonstrates the differing planishing efforts that will be needed for inaccurate and inconsistent gaps. For any inconsistencies in your weld seam (gap size, weld dot size, etc), keeping track of what, where, and how much is the tricky part, and adds to the challenge of sorting out the panel where it can be finished with minimal filler. Some may be OK in using thick filler, but it doesn't take that much effort to become more consistent in your processes to eliminate such a need for excess filler. This importance of consistency is in all the processes, starting with tight gaps at fit up. Everything that you can do to keep consistency throughout from start to finish only makes the planishing efforts more consistent throughout, lessening the need to keep track of the errant what, where, and how much.