Yes, you should do your gauge in the pattern stitch you will be using -- although some instructions will specify the gauge over stockinette. In that case you would do a swatch in stockinette pattern.
In order to get the right gauge for your pattern, you can change needle sizes. I would think that would make your life easier than adding stitches to make your pattern work out.
Too many stitches per inch, increase your needle size, too little stitches per inch, try a smaller needle than the pattern calls for. You may have to spend some time playing with swatches until you get the right needle size for your pattern but in the long run it's worth it to get your garment to fit right in the end.
I always hate the swatching part cause I'm eager to start knitting my project right away and dislike having to take the time to swatch. But I also hate having to frog a sweater midway cause I can see that there's no way it will fit!
So I swatch until I get it right.
Hope this helps!