maybe kind of a dumb question/concern but may I have your opinions anyway please as this is bugging me.

My technician of 23 years has faithfully tuned my piano and done other work on it. I have never contacted another tech until last month. My piano was sounding bad and I am considering selling it and wanted a manufacturer's technician's opinion. So another tech worked on my piano.
Do I tell my technician of 23 years that I had someone else do the tuning this time around, and why, or just let it go?
As a professional courtesy? Or is this even necessary?
Thanks.

'Not necessary unless you're asked for some reason. Tuning clients come and go. We get over it.

I think it would be courteous to mention it if you have your regular tuner come back. It will keep him from wondering what happened if the stretch or regulation or something is different that what he expects.

Better yet, it would be nice to let the new guy know it's a one time deal, if you intend to keep your regular tuner. That might keep you from having to turn the other guy down in the future....

If your previous technician of so many years is any good, he will have as much knowledge and probably more honesty than the dealer technician might have and should be fully capable of giving you an honest unbiased opinion on the value of your piano. We work on all makes and models. A prime example of this is a church that I have serviced for 35 years. I recently had given them an estimate on 2 of their pianos. They contacted the dealership. The dealership technician came over and told them that a 45 year old Everett studio was worthless. (One of the pianos I gave an estimate on.) Never mind the fact that I replaced hammers and bass wires on it less than 20 years ago, regulated, voiced it and all that, and that the piano really sounds very nice but needs more regulation and hammer filing after that amount of years and it is NOT worthless! They did this in the hopes of selling them a piano rather than fixing their existing piano which is very dishonest and very deceiving of the technician who's best interest was not that of the client but, that of the dealership.

Then, they told the church they could do the work for less. When I inquired further, stating that I have standards and will not cut corners or anything out of my estimate as what I quoted does in fact, need doing, the person at the church then told me that, the other tuner told them, that he could do it for less by "cutting corners and not doing all of the work that I had quoted them."

Fortunately, the church contacted me again and I straightened them out on the matter.