Either is fine.
Depending on what type of roving it is, it may felt a little if you dye the roving making it harder to draft.
Are you dyeing a solid color or multi-colored?
If multi-colored, do you want specific spacing on the final yarn? If so, dyeing the yarn might be better.
I dye both ways and do not have a true preference - it all depends on what I plan on doing with the final yarn.