Please, oh please, do not feel compelled to get gauge using the needle size listed in the pattern or on the yarn’s ball band.
This is one reason to keep extensive notes of your projects. You’ll be able to look back and see that for worsted-weight yarn, you almost always use a size 4 needle to get five stitches per inch in stockinette stitch (which is what I almost always need, even though most projects and ball bands recommend size 5 to size 8). If your next project in worsted-weight yarn suggests a size 6 needle to get 5 sts/in in stockinette, you can be pretty confident that your trusty size 4 is what you’ll really need.
I’m in no way suggesting you use your project history to skip making a gauge swatch, as annoying as making gauge swatches usually is. I’m simply making the point that knowing your typical gauge in various weights of yarn will help you get knitting faster.
What Will Happen When Your Project Gets Wet?
Why can’t you skip swatching? Because we can never tell how a specific yarn will react when it gets washed.
One yarn I absolutely love changed width quite a bit when I washed it. I swatched by casting on and knitting about 4 inches on the sleeve, to both give me a swatch and to see if the patterns were more trouble than they were worth. The patterns were OK, but the sleeve diameter went from 3.625 inches before washing to 4 inches after–an expansion of just over 10 percent. That doesn’t sound like much, until you realize that a 40-inch sweater would wind up being 44 inches around instead!