Knitting in the round may yield a different gauge than flat knitting. In flat knitting, the right side of the stockinette stitch is all knit with the wrong side comprised of purl stitches. When knitting stockinette in the round, every stitch is a knit stitch. For many knitters there is a discrepancy in size between a knit and a purl stitch which can impact the stitch count of your gauge. To ensure the accuracy of your gauge, the swatch should be knit in the same manner as the garment. For example, if you are knitting a sweater in the round then your gauge should also be knit in the round.
You can cut your swatching time in half with this short cut that will give you the proper circular gauge as you knit flat.