Gauge

I hate doing it, but I almost always do it. Even if it isn’t “required” in the pattern—which is often the case with crocheting, for example. With a foundation chain, you often can’t tell just by looking whether the width or length is right. BUT… I knit or crochet it, measure it, write down the results, and choose the needle size—done. I never wash them! Most of the time I’m working with standard 50/50 bobble yarn, so I know that (for me) it’s still accurate even after washing. With shawls, it doesn’t really matter ;-) But everything else… what are your experiences? Does your gauge change often? Should you really wash the gauge swatch and count the stitches again?

I think it depends on the material and the pattern. As you said, it doesn’t matter with shawls, but with clothing it can be disastrous. I once crocheted a merino wool sweater that was very tight before washing, but after washing and drying flat, it stretched out a lot—all the way to the middle of my thighs. With my last knitted sweater, too, I knew from the washed gauge swatch that the yarn would stretch a lot, so I was able to account for that. 
There are also many yarns that are coated to make them easier to crochet or knit with, and when that coating wears off after washing, the piece becomes much softer and stretches. So if you’re going to go to the trouble of doing a gauge swatch, washing it—and stretching it out if necessary—is definitely worth the effort.