I think the "depends on the song" responses are
almost on point, but I'd go a little further,
based on these conclusions I have reached about the terms ahead/behind/on top of the beat:
- its not a very accurate term for what is actually happening and confuses many (it confused me)
- "ahead" = feels rushed, but isn't really. "behind" = feels dragged, but isn't really
- every note of a song / phrases is not played entirely ahead/behind: individual notes within the phrase are displaced slightly, while other notes are spot on.
- thus, to play behind the beat , to play wat feels dragged, but isn't really, you displace some of the notes - usually upbeats or back beats- slightly behind the perfect grid. But the Downbeats are still on the beat. Thus you do not fall behind or lose the pulse.
So not only does it depend on the song, it depends on the
subdivision I'm playing
This understanding arose form an interview with John Paul Johns I watched, where he played examples of what "ahead" and "behind" the beat mean to him. I choose to accept what he demonstrated.