There's material early in the book that shows Tessio is not just a lesser character than Clemenza, but an outsider. There's no mention of Tessio at the wedding. When Sonny calls Tessio about the shooting, he tells Tessio that Mike is at college in New Hampshire -- Sonny would not have to tell Clemenza that. Tessio says something to Sonny like, "You know my boys, right?" again implying that they don't do business on a daily basis.