The more careful you are about not storing combustible materials near your home (and under your deck), maintaining vegetation near your home, and clearing up wind-blown debris, the less important enclosing your deck becomes. If you are going to store combustible materials under your deck, then vertically enclosing it with a noncombustible or ignition-resistant material would make sense. If you do this, make sure you have addressed any potential moisture-related issues by providing adequate venting. Enclosing your deck won't help if your biggest threat is an ember ignition on top of the deck, so make sure combustible debris is removed where the deck meets siding.
Assuming you have a gapped-board deck (i.e., when you look through the between-board gaps, you can see the ground), there are a couple of ways to enclose your deck. These include: 1) applying sheathing or siding around the perimeter (vertical enclosure), or 2) attaching sheathing and/or panel materials to the underside of the structural support members (horizontal enclosure). Sheathing around the perimeter is intended to reduce the possibility of a wildfire igniting combustible materials accumulated under the deck. Sheathing attached to the underside of the deck is intended to reduce the risks of a fire under the deck spreading to the rest of the structure. Except in the case of a solid-surface deck, a horizontal enclosure isn't recommended because embers that drop through the gaps in the deck boards may ignite debris that accumulates on the sheathing. If you have a solid-surface deck (e.g., lightweight concrete), it is probably already horizontally enclosed.
