I've still not seen Citizen Kane, so while I know what Rosebud is, I don't understand the significance within the film. Knowing it in advance isn't going to spoil my experience of the film.

Part of the point of Rosebud, in the film, is that you're supposed to start off thinking it's a mystery, and then realise at the end that it doesn't explain anything. Or, at least, not in the way you expected.