Your client can't be "wrong" in how they want something done. The best you can do (especially as a demo producer) is present other options they may not be aware of. Forcing a creative idea down your client's throat is worse than "following the herd", because 1) the client probably won't offer return business unless you wow them with your results and 2) you creative strong-arming might become synomomous with your name.
i have to disagree..well kind of. I know a guy who started as a demo producer and was very much a strong arming producer. He also really knows his shit, and makes bands sound a thousand times better then they would be without him. Because of this, his rep got big in ahurry, and he produced some bands that got signed directly because of his production and cowriting.
Now he only does label work and is making well in the 6 figures.
In fact every producer I know that really blew up and made a ton of money was always doing it how they wanted it, because they had a great vision and the bands/artists would benefit from it.
Disclaimer is that those guys are super talented and can outplay most musicians in the studio..and also have great ideas. If this is not you then PeeWee's advice should be taken.