As motorcycle owners, my mate and I often wonder when certain motorcycles with a reputation for reliability, go in for a "big" service i.e. valve clearances, do the technicians on the job actually do the job or just have a wipe around and don't bother.
Do the easy jobs, yes , but stuff that experience has shown don't really need doing?
If something did go wrong many miles later, then just charge it to warranty or bluff it out.
I know for certain that a long time ago when I had a Saab Turbo 16v S, bought used from a main dealer, the dealer put in warranty claims for certain things that had gone wrong but claimed for more expensive parts that were perfectly fine and they did not change.
I know because after a dispute with them, I wrote to the warranty company asking for a print out of the various work carried out and was very surprised that one of the items claimed for was a new ECU that was definitely not needed or changed and probably cost many many hundreds of pounds.
Ever since then, along with other incidents at dealers, fast fit merchants, I have a deep mistrust of main dealers.
So, yes, if they change the plugs, ask for the ones out of your car. Which then begs the question, did the come out of your car or did the just leave them in and give you some shonky others from somewhere else.