It's just like exercise, it's really good for you but nobody likes it. I personally hate making a test gauge I always think I can eyeball it. Well that system works sometimes but not every time. So "Rippit" lots of "Frogging" going on, but I just like the process and I don't know why but it does not bother me at all to rip a piece if it does not look right. A lot of hit and miss goes on, but there are some lucky accidents too. With all that said the bottom line is making a test swatch is absolutely necessary when following a written pattern as opposed to "knitting by the seat of your pants" to quote a friend from LYS. (Local Yarn Store for those who do not speak initials)
I machine knit as well as hand knit and when machine knitting it really is a must to do a test swatch. In machine knitting they suggest to cast on 40 stitches and 50 rows. Then to remove the cast on and pull and tug into shape and allow the square to "rest" and "relax" for a few hours. The poor yarn is in a state of shock after yanking it through those machine needles! If you prefer steam it or wash it so if there is any shrinking you will know. So in hand knitting I always knit 20 stitches by at least 20 rows. But the ruler thing makes sense I usually use the handy little Susan Bates "knit-check" Then measure for gauge, Elizabeth Zimmerman told me to always use a ruler when checking for gauge rather than a tape measure. She seems to think that it is more exact when a ruler is placed on the knitted piece rather then tape measure which can stretch a bit or move around. Also always measure in the middle of the swatch rather then the edges.
Converting a Pattern to fit to your gauge.
If the gauge you took is exactly what the pattern calls for go at it, start knitting. But if it does not match but you love the look and satisfied with the needle you chose you can convert the pattern. Here is how:
Simply divide the gauge you got by the gauge given in the pattern. For example suppose your gauge is 18 stitches to 4" but the pattern calls for 22 stitches to 4" Get your handy dandy calculator and 18:22=0.82
The pattern calls for 110 stitches with the 22 stitches = 4" and you are going to have 18 stitches = 4" so you need to cast on less than 110 stitches. By multiplying 0.82x110=90 you get 20 fewer stitches. So you cast on 90 stitches.