If you leave yours on the chain, then you're not giving him the best chance to be healthy or properly rested. A dog on a keep should be place in a quiet area by himself with no infestation, nothing on him(chains, weighted collars) so he could fully recover for the next days hard work. Usually what a dog on a keep does during the entire keep is work, eat, shit, and rest. Nothing more than that. The least you could do is give him his own private space. Now there are plenty of dogs that were on the chain during their keep and still won. But you can't deny the example I gave is better than to leave a dog a the chain.
Personally I think those who keep their dogs on the chain during a keep are rookies, or they don't give a f*ck if their dogs get infested with worms after they de-wormed them.