Kyle, your grandpa is a very wise man. There is no one size fits all when it comes to duck hunting across the country. There is no one size fits all day to day on the river here either. You can make generalities about what you most normally see, but those change fairly often too. Here, as a general rule on the snake river, most birds are in the middle of the river and mixed side to side loosely, most often staying away from the bank a good 30 yards swimming in reasonably swift water. You will find mallards against the bank a lot, but they usually will land 20 yards out and swim in. From a decoying standpoint for mallards you tuck them in against the bank, but have your divers and wigeons out at the 18 to 20 yard mark, so you do have visibility. Mallards will look pretty hard for someone who knows what to do with a call, a lot of times you don't even need mallard decoys. I've killed more mallards over goldeneye decoys than mallard decoys on the river. Mallards are far more inclined to look for sound than most other ducks.
Goose decoys are your friend, they work well for a lot of reasons. The reason we have lots of ducks swimming in the middle of the river is because of a debris line of moss that they feed in. That may not be present where you hunt. Birds will drift down the river getting tiny snails out of the moss, mostly this is the divers and wigeons, but they are all in the middle of the river. Usually there are as many as 3-5 thousand birds in these massive rafts, we traffic birds that are flying back upriver to reset their drift and it works very well. We use small spreads, maybe a dozen, usually less depending on what we are hunting. We have every duck you can imagine on the river and consistently kill 13 different species our of our hunting spot, even a wood duck on rare occasions.