I find it really depends on the dog. Some breeds are, of course a lot more greedy than others - I had a labrador that would simply scarf down anything remotely edible within reach, And even things that really aren't, up to and including homework, and a watch. My parents' border collie, on the other hand, knows to leave the groceries alone. Though, sometimes, she'll very carefully sneak up on the meat; but then, she edges closer so slowly, they usually notice before she gets to it. Hold a biscuit too long, though, and she'll assume you're offering it to her.