Deck staining - not necessary underneath? What about petrifying stains?

What kind of wood? If it is pressure treated you don't need to do underneath. If it is not pressure treated I would recommend an oil sealer...to keep the wood as breatheable as possible. If it is spruce, it has already had its useful life PS I haven't used the petrifying stains but I did do some research. If you are willing to pay the price it might save your deck. 50 dollars per gallon/ 100sqft for 2 by stock