There is one particular instance where you should not follow the pattern. This is when you make the "ribbing," which will usually involve a variation of the double crochet [British treble crochet] stitch.
If the directions say to use a crochet hook for the ribbing that is two sizes smaller than the one used for the fabric, use a smaller hook than that. Use a hook four sizes smaller, if possible.
If you are a visual learner, you might be able to read a pattern and know what the final result will look like. In such a case, you might also try deviating from patterns where you see fit—but be prepared to accept any ensuing mess-ups!
You can also, if you care to do the math required, deviate from a pattern to suite your gauge. If the pattern states six stitches per inch and you have five, or vice versa, you could try another hook to get the correct gauge, consciously adjust your tension to fit the gauge, or adjust the pattern to suit your current gauge.
Some design repeats require a certain formula of stitches, though, such as "add one to a multiple of five." If you adjust your pattern, be certain that you have the correct multiple of stitches to do the design you want.