Admit it; if and when you actually knit a swatch and block it, you mostly pay attention only to stitch gauge, not to the number of ROWS per inch. Now, a lot of the time that's OK, because in many cases once you get the width of a garment right, you can adjust the length of the thing as you knit. But it's easy to get complacent, and sometimes not getting an accurate row gauge can lead to all kinds of trouble. Here are some of the situations in which it's important to pay attention.
1. Sideways knits--sometimes. It's true that if you're knitting side-to-side (or centre-out or vice versa) row gauge isn't crucial so long as the pattern is based on knitting for a set number of inches. But, if the pattern is based on a set number of pattern repeats, it's another story. Your row gauge had better be accurate, or the finished size could be off--way off.
2. Garments based on mitred squares. When you're knitting mitred squares in garter stitch, the number of RIDGES (remember, two rows make one garter ridge) must equal the same number of stitches in order for a nice 90 degree angle to develop. For instance, in "Harriet's Jacket", you need to choose a yarn and needles that will work out to 4 ridges and 4 stitches per inch. The shape and fit are dependent on that relationship.
Be aware that some yarns, because of the way they are spun, will never work in this way. If you've tried several different needle sizes and nothing, after blocking, measures up to that perfect square, it's time to try a different yarn.
3. Some sweaters with circularly shaped armholes/shoulders. Here, you have to look at each pattern on its own to see if it is based on knitting a set number of rows, or whether it's based on a set measurement. In the latter case, there is usually some room for "fudging"; once the shaping is done, you might be directed to work until the piece meets a certain measurement. However, in the former case, as in my "Petrova" cardigan, the finished length is based on a set number of rows worked at a specific row gauge.
Changing to a lighter-weight yarn and switching to a larger size carries no guarantee of a happy outcome in this situation. It's best to stick with the recommended weight to get the size you want.
Note that raglan-styled sweaters are usually safe to make even if your row gauge is a little off. You can always add or subtract stitches at the underarm to make the body width work out. Top-down raglans, like the "Perth Cardi",
are forgiving, and a style I strongly recommend to those new to sweater knitting and fitting.