I expect there are people who can get the same gauge knitting flat and in the round, I'm not one of them. If not otherwise specified I would do an in-the-round gauge if the part of the project that is most important to meet gauge is in the round or flat if the more crucial part is knit flat. Say for a sweater knit mostly in the round, I would do my swatching in the round. I don't know what assumptions the designer may or may not make. Others will know a lot more than I do.
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper