G: Gauge-A-Z Challenge

Gauge or tension is the number of stitches per inch (or centimeter).  Gauge isn't as important when knitting a scarf or blanket, but it is very important when knitting sweaters, socks, or hats.
Every knitters gauge is different.  If several of us sat down with the same yarn and needles, we would each get a different gauge   Why?  Some of us would be loose knitters and get a gauge of 6 stitches/inch, while others would knit much tighter and get 10 stitches/inch.  This is a great article describing gauge.
A pattern will state the gauge  needed to obtain in order to knit the item to the specified measurements.  For example, in my pattern Summer Leaves, the pattern states that the gauge is 20 stitches=4 inches in stockinette stitch, or 5 stitches=1 inch using a sock weight yarn and US 3 needles.  I was able to use a US size 3 needle to obtain the gauge of 5 stitches for inch.  Another knitter may need to move up or down a needle size in order to obtain the same gauge.
Before beginning the item, a knitter should make a gauge swatch (more on swatches at S). I say should because most of us don't, we are in a rush to begin our garment that once it is completed the garment doesn't fit.
Do you check your gauge before beginning a project?