I clean mine every service, and may even check it between services [by taking the intake hose off],
have taken mine off numerous times and cleaned it, and of recent do it while on the car by spraying degreaser on a rag, and opening the throttle by hand, and cleaning it that way.
Yes its another money milking option, as they don't take it off, or they just open the throttle and do it with a rag, and charge you an hours labour for a 5 minute job.
I know, because in the early days, I use to set my car up to see if they actualy did what they say they do,
I marked a TB bolt with liquid paper on the side of the bolt head,
So if they attached a socket to it, it would mark/damage the liquid paper on the bolt head or Nut, or the bolt/nut could be replaced into a different position,
Well, that's were they came unstuck, when I went to pick up my car, I checked under the bonnet and noticed the bolt/nut had not been touched with a socket, as I did it up real tight, so the use of a socket would of damaged the liquid paper,
I then challenged them about "All the items that was listed on the service docket if they were done", and they claimed that they were all done, I asked them "are you sure", they said that its all been done,
I asked them, how they do throttle body clean, they said they remove the throttle body and clean it, and place it back on the intake,
I then pointed out that the liquid paper on the bolt I had marked was not damaged,
They said its been done,
I then said that's interesting, because my marking has not been damaged,
they then asked the Technician, and he said that he didn't do it,
As he just thought it was a Standard minor service.
In a nutshell, and ever since that error of their ways, I do it myself, and save my hard earnt money.
Side Note: If it is real dirty, remove it and clean it,
Then just check it now, and then and use a rag with degreaser on it through the mouth of the throttle, and it will always breath easy and idle smoother.