Ideally you don't want the lumber to be over 19% moisture content.

If you are talking about saturated treated stock I would suggest you wait, the attachment to the house is one of the most important connections on a deck as it typically supports half of the deck.

There are a number of reasons. The bolting of the ledger to the house will loosen as it dries, you want those nice and tight and solid.

The added end treatment you apply to your cut ends won't soak into the member to a decent depth if it already is saturated with moisture.

If you have siding then the locked in moisture between the flashing, wall wrap and ledger may create problems to the surrounding untreated stock. Dependent on your area and insect nature.

If you are adding a air space behind the ledger then again you may have problem with shrinkage.

So I suggest letting it get under the recommended moisture content.. I know there is no way to tell if you don't have a meter. So take a clamp and tighten it up and if you see water beading up at a significant rate (that will have to be your call) then I would let it air for a little longer.

Keep it stacked nice and level and straight while it airs with stickers every few feet so it won't warp on you, and out of the direct sun if you can.

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