Well, it is possible, but difficult, as you have discovered. Yes, sometimes the yarn is marked DK when it's really worsted, etc.
I typically attempt to match the stitches per inch as closely as possible. But I don't really concern myself too much over the rows per inch. I figure that I can adjust the length by measure easier than I can by row-counting...unless it's something very specific, like shaping an armhole...in that case, I try to make minor adjustments as best I can until the shape/length match up as best as humanly possible.
After all, I'm merely human...So, don't beat yourself up over this!! You are too!