It really depends on what you like and what your dog is like!
My current cavalier, Kokoda has very short hair. He is 16 Months old and his hair is rather thin and therefore appears shorter than it is.
He looks like he has been trimmed because his hair lies flat along his body, and isn't fluffy. It is more smooth and sleek and doesn't shed!
I did trim his ears a while ago as in the summer he was swimming almost every day in the creek and i was worried about ear infections, so i trimmed his ears so they were lighter ans had more air flow. And an added bonus that It also kept them out of dirt and food!
I am a groomer so i did it myself, but i did trim my previous cav. He had very thick fluffy hair and being tricolour he got very hot in the summers.
He also got rashes on his belly if he got too hot and that was very uncomfortable for him.
So each summer i would shave him rather short but i left his ears tails and the feathers on his legs. Another added bonus is that he shedded slightly less and the hairs he did shed were shorter!
To those who say that it changes their coat if you shave them, it never changed his in 7 years of doing it.
each winter he would grow back to being fluffy