Most times row gauge, won't matter, but there are a few times when it might...
knitty article on swatching:
"Row gauge is absolutely critical if you are knitting any raglan garment or a fully-fashioned sleeve, if the pattern is Fair Isle, other colorwork, or certain Aran designs, where the charted design constitutes the entire garment piece and dictates the finished length. Always analyze your pattern to see if row gauge is critical to the finished garment or not. Most of the time row gauge will not matter too much but make sure you know when it does."
If you need to adjust your row gauge, this
may be of help. Lucia also discusses row gauge in greater detail here