There are two things to understand about wood grain: how it works, and how it looks. But sometimes it ain’t easy to consider both in the same project.
Wood grain is important when building anything, but the reasons are sometimes mutually exclusive. The mechanical properties of grain are, by far, the most important – arrange grain incorrectly and you’ll certainly have issues with joinery strength, warpage, expansion/contraction, stain or other finish absorption, etc. At best the issues will be small; at worst, your project will fall apart.
The aesthetics of wood grain are also important, but entirely for reasons of how much you like the way it looks. You can make a thorough search through lumber racks to find the grain that looks the best and matches the best, or you can go with the luck of the draw. There are merits to both.
The mechanical aspects of grain aside, I tend to split the difference with grain appearance. I won’t use stock with obviously mismatched grain that would look terrible, and when I’m laying out stock for cutting I’ll always try to match things up when possible. On the other hand, I don’t go crazy with it, because sometimes even the most exhaustive attempts at grain arrangement are thwarted by the grain itself.
I just finished a small box for which I did work hard at grain-matching for the sake of appearance. The project turned out very nice and will make a fine gift this Christmas. However, once I applied the finish that “perfectly matched” grain turned out to be anything but. Even though everything looked exactly the same during building, once the finish was on one component of the project was far lighter than the rest of the box. In the end I had to strip that side of the box, do some creative staining and then reapply the final finish. I got the even look I was going for, but it was a huge pain.
After decades of woodworking we like to think we’re pretty smart, but sometimes all our experience comes up short when the wood shows us that it has a mind of its own.