Every since this post, gauge has been on my mind. At the shop, we always encourage you to swatch. For a class, we go so far as to insist. Beginning knitters often don't know what it means to swatch so they just don't. Or they think, "Oh, well, it's just a scarf -- I don't really need to swatch." Some knitters, especially those with a few years under their knitting belt say, "I'm always right on gauge." Even very experienced knitters have a tendency to look at the pattern's gauge and think immediately of what size needle they need to get that gauge, and off they go. It's tempting to go along that way, especially when so often you find that you are right on gauge. Lots of times it works out fine, but there are plenty of situations when it doesn't.
For instance, some yarns need a much larger or smaller needle than expected to get a particular gauge. That's the case with the yarn I mentioned in my last post. Cotton Lustre is a worsted weight yarn with a native gauge of just over 4 stitches per inch. But the ball band and all the patterns designed for the yarn recommend a size 10 needle (6mm). Size 10? You might be tempted not to swatch but just start a project with the needle you typically use to get that gauge. But you would be very sorry, because that beautiful sweater will be much smaller than you intended Here's why. Cotton Lustre is really bouncy. As you knit a bouncy yarn, it stretches around the needle and then springs back into a smaller stitch once the stitch is moved off the needle and lives as part of the fabric.
The reverse is true with a yarn that lacks elasticity. Standard construction cotton, silk, and rayon yarns have very little elasticity, so you need to work them on a smaller needle. The smaller needle gives you a firmer fabric and compensates for the fact that the stitches are not going to bounce back and create their own tightness in the fabric.
So, the point of my ramblings here is that even though you're a good knitter and a smart person, some yarns behave differently that we expect, so you really need to swatch them. See how they behave and what you have to do to get the gauge you need and the fabric you love. Plus swatching helps get you familiar with your fiber and its idiosyncrasies. Consider it an investment in the success of your project, and you'll find that swatching has a very high return on investment!