When choosing furniture for a client one must enter a very selfless state. It is not about what my personal tastes are - if an artist who likes to paint horses is commissioned to paint the King's Daughter he would be well served to focus on the subject at hand and not the horse who innocently grazes in the background! This is a skill that needs to be learnt quickly in Interior Design (and is perhaps the fundamental element of it) seeing as tastes alter so radically between people.
As a Designer I like to take risks - I would much prefer for someone to feel that I had gone too 'over the top' as opposed to having been too conservative. Having said that, it is vital to pick one's spots: too many 'showcase' pieces will drown each other out and what will leave the room resembling a grenade fight in a paint factory. Highlight colours should act like commas and punctuate a solid piece of artistic work: every piece should be chosen with the other pieces of the room in mind.
I believe that this is the main reason why people use Interior Designers: anyone can see that a certain sofa is funky or practical (and the same goes for many individual pieces of furniture) - the skill is in weaving pieces together in a seamless manner. Great Design, in my opinion, is essentially about being able to take a step back and be objective whilst not neglecting the details.