Like I said yesterday I am a big advocate for swatching. It is really important whether you are designing your own knit item or using a designer's pattern. Everyone's gauge is different and swatching allows you to see if you need to make any adjustments to the needle size or stitch count. Swatching also lets you get to know the stitch pattern you are going to use and lets you experience your yarn and get to know it before you take on your project.
There are plenty of reasons to swatch and today I am only going to talk about one of those reasons, stitches per inch. When you knit a swatch you want to make sure that the swatch is about 4 to 5 inches in width and height. If you are using a pattern there will be a suggested gauge of usually 2 to 4 inches you want to knit about an inch to two inches more than this suggested gauge so that you can get an accurate measurement. If you are looking to find a certain yarn's stitch per inch look at the ball band, there is usually a suggestion of an amount of stitches to 4 inches or to 10 centimeters, this is a good place to start. If a suggested gauge is 20 stitch to 4 inches when you cast on for your swatch you will want to cast on about 30 stitches.
Now if you were using a pattern and you had too many stitches per inch you would go up a needle size till you got the right count. Inversely if you had too few stitches per inch you would go down a needle size till you got the count you were looking for. It makes a big difference if you are 5 stitches to the inch or 6 stitches.