As a rule, if you feed good quality dog food, you won't need to add supplements for coat in your Labrador's food. Their coats are naturally oily, and a good coat is determined mainly by genetics. In other words, if the parents have a good coat, then it is likely that their get will also have a good coat.
That being said, I have added Mirra Coat to the food of our Labradors for years. It has amino and fatty acids and the dogs love the taste. I've also added salmon to their diet on occasion, and they loved that. I don't know whether the dogs would have had good coat regardless of these additives, but I suspect that they would.
I can say that the dogs have a nice oily coat, very little dandruff (except during shedding), and their skin is good. I tell puppy owners about Mirra Coat but if not fed, it probably wouldn't make much difference. It's a funny thing about dog foods and supplements though. Everyone will have a differing opinion. If something works, then why change it? So if your dog's coat looks great with just the kibble that you're feeding, then I wouldn't rush out to buy any supplements.
One of the supplements that you don't want to ever use is calcium. I can remember when people would talk about adding calcium to their dog's diet. It is actually deterimental to development of bones in Labradors, because excessive calcium intake can result in hypercalcitoninism and hypoparathyroidism that will actually retard bone maturation, inhibit osteoclastic activity, and will slow cartilage maturation. These effects on bone and cartilage increase the incidence of osteochondral lesions in articular and physeal cartilage, thereby contributing to the incidence of OCD.
Calcium is sometimes given to near term bitches at low doses to help during whelping. I don't add calcium except for about a week before whelping. Yogurt is another good source of calcium as is cottage cheese. Calcium in the form of Pet Tabs can be given safely after whelping to provide supplementation of lactating bitches.
Another supplement that has been reported to help prevent hip dysplasia is vitamin C. Studies have found that excess vitamin C supplementation is generally considered to have little or no effect on the skeleton. The relationship between vitamin C and developmental disorders of the skeletal system in the dog is as yet unproven. No valid scientific studies have demonstrated a positive effect of oral supplementation of vitamin C in preventing CHD in growing dogs that are genetically at risk for the disease. The relationship between vitamin C, joint laxity, and CHD in the dog is suspect because a decrease in systemic vitamin C levels could be expected to affect other joints.