Bob and I always laugh about the irony of how the "better" or more successful you get, the less you usually have to do to the source mixes because you end up (usually) getting better and better mixes.
I think we generally tend to get a pretty decent level of source mixes, but every once in a while you just get those big turds and I think it's good for me. In most cases, the clients are aware that it's not the greatest sounding thing in the world and are stuck with it for various reasons. Most times they end up really pleased that it did go from a "D" to a "C" and hopefully they pick up a thing or two to make it better the next time. You usually have to learn a new thing or two in these cases I find.
I have twice sent clients back to remix when I knew that they were home recordists who had ready access to the equipment and time. It did make things at least somewhat better in both cases. I have never turned anyone away because their mixes weren't up to snuff. Though that sounds like it'd be nice, I don't think it'd be a good business move for a facility that caters to a lot of independent musicians on limited budgets.
Jason Ward
Chicago Mastering Service