There are many so-called rules in woodworking. Many of those rules are simply guidelines. (Yes, there are differences.) The more experienced you become, the more you understand when you can bend the rules and when you can snap them like a twig.
The procedure to four square a board using woodworking machines is relatively straightforward. Flatten a face at your jointer, establish a thickness through a planer, square one of the edges back at a jointer then rip the board at a table saw. But there is a time when step 4 could be left out of the mix.
I watch as woodworkers in the shop, in classes and online prepare boards to glue into panels. Invariably, each board is four-squared before it’s ready to assemble. There’s a wasted step in that process. Sometimes, especially on a two-board assembly, you do not need to make the table saw cut that parallels and squares the second edge (step 4).
When I prepare boards for a glue-up, I first mill the faces flat and parallel. I then lay the two boards out and choose the best grain match, then mark the two edges I intend to join. Back at my jointer I run the two marked edges, then assemble my panel, leaving the outer two edges of my boards rough. (You can do the same in multiple-board assemblies, too – with three boards, align and mark the four joining edges, again leaving the outer edges rough.)
Why? It’s not necessary to have the seam in your assembled panel running parallel to the two outer edges. Plus, you may need the extra 1/8″ or more of material you cut away to make the final width of your panel. And if your boards are narrower on one end, which many boards are, your arrangement may align the narrow end of one board with the wider end of another so that the width of the panel actually grows.
In final preparation of the panels, choose the outer edge that is best – less work to flatten and square – run it over your jointer, then make the step-4 cut at your table saw. That last cut is done once. You save time and, potentially, money.