I've never heard or read advice like that either. I don't think dogs would notice/care if their food were spit in, but it feels disrespectful and wrong, not to mention gross. On top of that, I am sure their food already has my scent on it without spitting in it, especially treats that I give them by hand. I think what is more important is that a dog realizes food comes from you and that you decide when and where they get fed.

Better things to do to show you are a higher ranking member in the pack is eat first and then feed them, go through doors first, and general obedience exercises with positive reward. When Tasha was a puppy and when we first got Tommy, I also handled their food or would reach down pick up a bit of food out of their bowls while they were eating so that they accepted I could do that.