Speaking AS a contractor who has been working on the client's site for the past 9 months or so, I believe I can speak from both perspectives.
When I'm back at the office, we've got about...let's say 5 or 6 major clients and it's a bit of a struggle to keep up with everyone's standards...especially when the clients struggle to keep up with their own standards!
To be honest, there shouldn't be any need to create a separate set of standards, but I think the key is to make it as simple as possible for everyone to follow them. It's kinda like CAB's sig..."An extra dollar spent on "design" will save ten dollars during construction." A little extra time spent smoothing over the standards will (hopefully) pay off in the long run.