I always read the manual. There may be some tidbit that isn't covered in the game tutorial, or other useful information. At the very least, I'll know what it contains in case I get to a point in the game where I think I need to know more or refer to a table or list.
One thing that always infurated me when I was a kid was the tendency for people to not read game manuals. People would come over to my house, plop down in front of the Intellivision, start a game, and then start nagging me with questions. "How do I do this? How do I do that? What can I do here?" I'd reply, "There's a manual right there, READ IT." "Oh, that's too much trouble, I can just ask you." AGRGARGHSGHSDGHSDHGSDGFWEGJBDFHQERGHWEFSJBDVBWEFGK DIE