Full disclosure: I am a decking contractor. I also have zero experiance with hemlock as it is not a species available where I live.
That said, hemlock is rated low for durability and susceptible to insect attack http://www.wood-database.com/lumber-identification/softwoods/eastern-hemlock/
Consider the difficulty in replacing one of these members if it were to fail. Low level decks are already high risk as far as decay and insect damage goes and In those situations I always specify the best quality product available. Reason being, if one of the decking boards fails it is an easy repair to remove the offending board and replace it. But if any part of the sub floor frame fails it means demolishing the deck to get to it. Especially if there is zero crawl space.
I'd suggest that the wisest place to spend your money is in the framing.
With regard to low decks in general.... Drainage, drainage, drainage. Consider areas where water will pool and allow for drainage to reduce the likely hood of your foundations sitting in permanent puddles.
Hope this is of some help to you