A webdesigner has more to do than make the client a site, he should discuss with him what he wants, if the clients asks you to do something you know better isn't done , or much better done otherwise you have to tell the client, you have to have an agreement with the client that you do whats best for his site, you are the webdesugner, he's not its only normal you know things better than the client, and the client will understand that you know what isn't such a good idea.
thats like a sculpturer, if he has to make a statue, and he has to make something wich is a bad idea, something that will break easely or something, he will tell the client about it.