How does your trainer know she's going to be a big dog? I hope he/she didn't say by the size of her paws lol.
If the dog is bred right and isn't overweight, I'm not sure why additional joint supps are needed.
For what it's worth, I have a close friend who is a scientist and works in R&D...there have been studies done on joint care for humans which show that there isn't a direct link between taking glucosamine/chondroitin and it doing anything for the joint itself. In reality, a healthy diet will trump any "supplements" someone can take, human or animal.