Yep, write down what you want done. You don't need to get the dog clipped at a groomers if you are thinking this is the norm -- it isn't for cavaliers -- they will also just wash and dry, trim nails etc.
What they are likely to do if not given explicit instructions:
1) shave all the hair (slippers) off their feet
2) trim their tail into a weird brush shape
3) cut all their leg feathering into a sharp edged cut
4) cut their ears all the way back to the leather after you spent years growing them
5) shave them down completely :shock:
6) trim them like a cocker
If you do not want these things done (and some people prefer one or another of these elements for various reasons), then tell them to ONLY lightly thin the coat, and leave ears, tail, feathering, feet totally alone, put this in writing, and restate it about five times. And even then -- expect something you don't want because they will decide you really must have meant you wanted the ears trimmed/feet shaved/whatever. Groomers seem to find it very hard not to shave or trim something on a dog -- the concept of a wash and blow dry or a dog that doesn't need to be clipped like a hedge seems alien to them. :-|