The dealer is absolutely correct the throttle body on fuel injected cars gets carboned up and must be cleaned every year or so. This because unlike a carberated car there is no liquid fuel running across it to keep it clean. No amount of fuel cleaner can fix this.
If don't do this the car will idle roughly and the off idle response may start to bog. It makes no difference at all for partial and full throttle performance but it feels smoother.
The carbon builds up a layer and some time a ridge where the throttle plate touches the bore. You just need to clean this off.
It is a simple job to do yourself. Just pick up a can of "throttle body cleaner" spray at your local automotive store. Disconnect the hose going to the throttle body and soak the throttle plate with the spray. (The good stuff is in a spray can designed to work upside down so you can spray inside on the plate.)
Let it sit for an hour, put the hose back then start your engine for a few minutes. Then repeat the soaking until you have gone through the can. Check your progress with a mirror and a light.
If you are a clean fanatic like me you can unbolt the throttle body and clean it better on the bench with the spray and an old tooth brush. Don't attempt unless you are mechanically inclined. (I hope for $189 that is what the dealer does.)
The solvent will also clean some carbon off the inside of the intake chamber and maybe cylinder heads but it is the layer that sticks to the throttle plate that is critical.
PS: If You really have no proficiency with an engine (ie Don't know what a throttle body is). (We all have different skills.) Shop around, another mechanic may give you a better deal since this I cant see this involving more then an hour of labour.