Installing new deck boards, because of the chemicals, I would prefer to wait until September to stain the top of the decking letting the wood to dry. But would you stain the bottom to help preserve it prior to installation?
That's what I've always told my customers. When it's a high deck where the underside is easily seen I'll spray a light coat just for appearances.leaving the underside raw will give any moisture that enters the wood a means of escape, so keep that in mind too.