It depends on how you stock your tank, although some may disagree with me, others may agree. If you have a small group of shoaling fish then i'd say you can get away with not vac'ing it as it's used as a plant fertiliser if you have enough plants. There was a topic about something like this a while back, and lots of people had different opinions. If you post your stocking it may give us a better idea on how to supply you with an answer.
Neal