Using a 1/4" per foot grade sounds good to me for a sewer line, but not a deck. I do give decks some grade away from the house, maybe 1/8" per foot if a short deck, and 1/16" per foot on a longer run, say 10 ft. or more. This will affect the appearance of a deck's surface a lot. As far as the gravel under the footing blocks: I would do that, this promotes drainage away from your blocks instead of allowing water to stand around them. Actually we set our deck post in concrete with gravel at the bottom around here to promote drainage, leaving the concrete about 1" above grade and tapered to allow run-off from the post.