Get rid of it before it breaks down and ends up between your deck boards
If leaf or tree mulch gets stuck between your deck boards, it will eventually turn into an organic mud, like what happens in gutters which, then holds moisture and the rot sets in.
We have recently serviced some decks on a holiday house where this has happened on an older hardwood deck. The mulch caused the boards to rot out from the edges of the boards. So, the top and bottom of the boards still look ok but, the middle is spongy and rotten. Keep the mulch out of the gaps between your deck boards and your deck will last a lot longer.