I personally believe that every single large breed dog should be on glucosamine from 4 months of age on for life. Big dogs need all the joint and cartiledge help they can get and since glucosamine is far better at "maintaining" cartiledge rather than rebuilding it, it must be used preventatively for best results. My vet and orthopedic surgeon both agree that big dogs should be on this.
If there are joint issues then I also strongly recommend hyaluronic acid - I specifically use Synthovial 7 since it is in liquid form and more readily assimilated by the body. Hyaluronic acid is an all natural lubricant found in the body, and when lacking, is responsible for joint issues/arthritis, vision loss, and skin wrinkling. If you are unfamiliar with it, definitely do a search on it and learn more.
As for supplements - the worst advice you can get is someone telling you an absolute - raw fed dogs don't need any or raw fed dogs must have them all. There is no such thing that can make such statements accurate. The fact is, supplementatio n may be very needed or not at all - it depends on the individual dog, its genetic history, your geographic location and where your food comes from and any specific deficiencies your dog may have. I can say that unless you live somewhere where you can feed fresh kill - skin, fur and all, or a few unique places in this world, none of which are in the U.S., that do not have depleted soils, then some supplementatio n to some degree will be necessary for a complete diet. I am concerned with how many people I encounter who are buying into the notion that they can feed a "prey model" diet, but get the food from their grocery store which is corn fed meat and vegetables grown in serious depleted soils and yet they want to believe their dog is getting all of these vitamins and minerals that if they actually tested, they would find are not even present. It is a fact of life in the U.S. and a lot of areas of Canada that our soils are so depleted, that necessary trace minerals are rarely present in any of our meat sources, even when vitamins are. As a result, the most credible animal nutritionists including Kymythy Schultz and Pitcairn agree that dogs must be provided with kelp, fish oils, and vitamin E and in many cases alfalfa or they will be minerally and EFA deficient. Before anyone ventures into a natural diet, I strongly recommend they do sufficient research first as there are many views out there - start with Kymythy's book - Natural Nutrition for Dogs and Cats....it is a great starter book.