Honestly, I think that as long as the dog is healthy, it doesn't really matter. It's like people, some of us will digest some stuff more or less well, or get sick when they eat something else. If your dog is too skinny no matter how much he eats and that his coat isn't shiny, by all mean, change their food.
I keep thinking that for dogs when you see the crap they eat outside, it can't be worse, lol. Seriously though, the only thing that really irks me is people claiming that their food is good when it's really not that great overall. It just comes up to people being misinformed, as usual. But food made from real meat will always be better than cereals and lots of the crap that some foods use nowadays. But when it comes to dogs, it doesn't mean it's not 'overdoing' it (like I am 100% convinced that feeding organic is often overdoing it for humans).
Manchesters, I'm surprised that you think that pedigree is ok and science diet is crap though, I've always thought that SD was better...