If you’ve ever been involved in a deck project you’ve probably come across the bark side up or bark side down debate. 

Some claim that a board wants to cup towards the bark side which will hold water and rot quicker, while some say that heartwood (pith side) rots more slowly, and still others that sapwood (bark side) will absorb the sealer better

Ultimately, relative (and totally variable) moisture content is going to have its way. Just put the best side facing up, securely fasten the boards, and clean/seal regularly.