My guys always yawn when they wake up. I don't think I've ever seen them do it any other time, though.
As a side note, yawning in dogs can also be a sign of stress and even an impending bite if the dog feels threatened. So don't assume that a yawn from a dog means he's relaxed, take everything into consideration and always be careful. I know my dog always yawns continuously when he's stressed (like at the vets or groomers). When I worked as a groomer, I spent the whole day yawning just from watching all the dogs yawn.