Swift swatching in the round
There are many ways to knit, but whichever style you work in one thing remains constant: knits and purls are never worked in exactly the same way. Consequently it's very common for knit and purl stitches to be slightly different sizes. This can cause a range of visible tension inconsistencies and next week's post will cover dealing with those. It also means that it's common to find a difference in gauge between stockinette worked flat (with purls) and stockinette worked in the round (no purling).
Swatch the way the project will be worked
A good general rule and in this case that means swatching in the round if the project will be worked in the round.
The problem with small tubular swatches
Beginning with a sleeve is often recommended as a way to avoid swatching for a sweater but I don't think it's a very reliable method. The reason is that sleeves are smallish tubes and you'll be using magic loop, dpns, or circulars with shortened tips to knit them. The needles end up being held fussily, you're pulling the yarn tight to avoid ladders at the joins, the odds of your gauge being tighter than it normally would be on that size are high. I often go up a needle size for sleeves in order to match the gauge on the rest of the garment.
The problem with large tubular swatches
Seriously — they're large! If you want to knit a 14-16" circumference in order to have a swatch that fits around a circular needle (and arguably you should go even larger so that it fits on a needle with regular length tips) go right ahead. I do not want to do that.
The smarter way to swatch in the round
It's not knitting a tube that matters. The crucial thing is whether or not you're working wrong side rows. To work a swatch with no wrong side rows cast on and work the first row on your circular needle. The stitches at the edges will be messy so cast on a sufficient number so that you can measure 4" / 10 cm in the middle.
Instead of turning the work for the next row slide all of the stitches to the other end of the needle, just like making an I-cord.
Bring the yarn around the back of the work and work across with right side facing. At the end of the row slide the stitches along the needle again. Continue in this manner letting the yarn trail loosely across the back.
If you find it difficult to keep the strands loose just let them tighten up. You can cut them to get the swatch to lie flat.