We intend to build a 10'x20'deck with 2x8 pressure treated framing and 5/4 pt decking.
Is it recommended to install a 6 mil poly layer under the deck?
No reason to do so. As long as the 2x8 (which likely is not ground contact) is not touching the ground, then you will gain nothing by adding the poly layer. In addition, what you will do is create low spots with the poly that will hold water - causing mold or even insect problems.
You can keep the under deck surface dirt, or add a layer of stone before building the deck.
To add to Jaybee's excellent advice, you might consider laying landscape fabric down under the entire deck area and then cover it with a 2" layer of stone. This will allow water to soak into the ground and also prevent any weeds from growing under the deck for many years to come.
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