I wouldn’t want to hear the Imperial March before its first appearance in The Empire Strikes Back. Having it hinted at here is the right way to do it.
But why would you watch this movie, or any Star Wars prequel, before The Empire Strikes Back? Logistically speaking, this movie came out 36 years AFTER The Empire Strikes Back, so there’s no way anything in Rogue One happens before it.
All that nitpicking (admittedly) aside, I do agree with the sentiment. But since Rogue One is a sequel (prequels are sequels too), I don’t necessarily see a problem with using production elements such as music when appropriate. But I do like when they weave those things in narratively. As has been mentioned, Anakin’s theme in Phantom Menace is a variation on the Imperial March, and that makes perfect sense. It’s using the music as a kind of backwards foreshadowing. I’m a little bit more on the fence about its necessity in this case. Anakin’s Theme in TPM makes sense because he’s not Vader, so a gentler, happier version of the same motif makes sense but also ties him to the same musical theme. It helps illustrate the progression of his character. But here he’s full-on Vader. There’s nothing between here and Empire that significantly changes his character. It’s just that John Williams came up with a theme for him in between the first and second films. A hidden Imperial March would suggest that there’s some kind of Vader that’s still covered up and is trying to emerge. But here he’s force choking people and slicing and dicing by the truckload, so there’s really nothing to uncover. So narratively-speaking, there’s no real reason why it would be out of place for him to be accompanied by that theme.