I would definitely swatch in the round-- your gauge can be very different from flat knitting. Here’s what you do:
Let’s say you’re trying to get 18 sts over 4"/10cm in stockinette stitch.
Cast on about 25 sts on your circs and knit them.
Then, slide the stitches to the other end of the needle, loop your yarn around (make sure to leave a very loose loop in the back so that it doesn’t affect being able to lay your fabric flat) and knit the next row.
Keep doing this until you have several rows and can get a good sense of your gauge.
This is a little messy, in that you have loads of these long loops in the back, but it’s really worth it because you are knitting every row, just like in true circular knitting, and will really get an accurate gauge.