Designing for clients is fun and challenging, but one of the downsides of designing [for me] is responding to tricky comments and questions from client feedback on the designs that I have created for them. I want to be honest with clients – I don't want to devalue my services and my knowledge by giving into poor design choices. On the flip side, I have to be careful in how I deliver my response, so as not to offend a client.
Eric Karjaluoto from IdeasonIdeas.com wrote an article on just that and it's worth reading here – "How to disarm 10 difficult client observations / requests". His approach is helpful because as a designer you remain in control of the design direction, yet the client should be happy that you are looking out for their best interests.