I guess none of have bought a new tool and wanted to see if the cast iron tables are flat? Or how do you set up the infeed and outfeed tables on your jointer if you don't have a good straight edge?
Yes, and I created my own thread about it, searching for a good straightedge to do the job. See:
I was very careful to flatten and measure the flatness of my SawStop table saw. I freely admit that I invested more time and effort than was necessary, but I treated it as a once-only task. It was personal quest that was important to me. I did not expect others to agree, approve, or care.
I bought an Empire model "4004 M" ruler, which was also labeled "Straightedge". It did not cost much. There was no easy way to test it in the store. I tested it for flatness on a flat surface and it was perfectly straight on both sides, at least to whatever precision the flashlight test will yield. I used it to optimize the flatness of the table saw.
It was a valuable learning experience. I learned how much deviation there is in my table saw, and I learned how much 1/1000 and 10/1000 inch really are in terms of visualizing the gaps. Sure, I can look at and touch feeler gauges, but it is not the same as seeing the light shining through the gap (and measuring it) to truly understand what those measurements mean.
It gives me pleasure to use my well-flattened table saw with its nearly invisible seams in the cast iron, but that's me. Many people here would say that I wasted my time, but I gained experience and knowledge that I can use elsewhere.