The adjustment is sometimes necessary, because rotary cut pieces are usually slightly smaller than their template-cut counterparts, due to the absence of marked lines. That narrow pencil line we used to trace around templates wasn't very wide, but most of us tended to cut on the outside of it, adding a bit of excess. The width was often just enough to compensate for the loft that gets lost in a seam when it's pressed up and over the thread.
Think of that missing line whenever you measure strips or sew seams. Accuracy takes practice, but it won't be long until you know exactly where to cut and where to sew to achieve the best results.