In my opinion, home cooked meals or store bought bags or cans... you should still add a multi vitamin. I've personally seen too many improvements in health to NOT supplement.
That said, please make sure the viatmin you are giving does not contain the artificial sweetener Xylitol. It can cause dangerous spikes in glucose/insulin levels and can cause liver damage. Just 3 grams of the stuff (or 8 pieces of sugarfree gum) can kill a 65 pound dog.