I agree. I would not compromise my own integrity--personal or professional--to make a client happy. That said, I'm not sure it's an either/or proposition. I think I can remain professional but just present it in a way that is more comfortable to them...which, really, is part of being professional.

As an example...when I got there, we had a big meeting with me, K, and all the dev/tech people. K asked "is there any way that we can improve the accuracy of our estimates and/or development processes?" No one responded for several very long seconds so I put my hand up and started talking about Agile Development.

No one in the room had heard of it except one guy, their prime developer. He apparently has a mad-on about the idea, because he shut me down instantly.

I said "ok" and shut up. Then I started tracking my project in iterations, talking about velocity and user stories, etc. Other people are starting to pick up the language and adopt the concepts--and all without making anyone uncomfortable.