Olive Knits patterns generally call for swatching done flat.
Marie has always typically swatched for her garments flat. That being said she has also mentioned that a swatch in the round would be more accurate for a pattern knit in the round, but she does not do them herself in the round. In the end, it is a personal choice. You can always do one of each and compare them. It would be a great way to see how your gauge is different between the two methods.