I've helped a local company do their web site over the years. They have come to me several times with 'great ideas' for their web site. From 'nice' blinking text to hiding text at the bottom of the page to 'fool Google'.
When they come to me with these idea's I fill them in on why that is either a bad practice or a downright bad idea.
And this isn't just this client that I've had these kinds of conversations with. Most highly respect my technical skills and accept my answer. Sometimes they listen to my reasons and decide differently. In the end they are the boss. I've only had one client that seems to constantly override my thoughts, frustrating for sure.
But this local company came to me with another 'great idea' and I again told them why it was bad. So they decided to go with somebody else. Honestly, kind of relieved. But I wait for the day that they face some of the consequences of not having somebody willing to tell them no. When Google's spider realizes the hidden text and drops them from their index.
So I guess the bottom line is, feel free to tell a client no. But realize they are the boss! And if it is in your power, and them telling you no just frustrates you to no end... drop them as a client. Having them make you design a bad site or application isn't worth your reputation.