My former teacher used to send new instruments to a professional to play them for a week or so before giving them to students. He said it was to tune it. However, I can't see how a metallic object would change permanently after blowing warm air into it.
I have a picture of a professional player continually 'blowing in' new instruments on his gigs. Not terribly likely, is it? :-)
You want an anecdote? I bought my first professional level trombone from Denis Wick, at the time when he was still primarily principal trombone in the LSO and just beginning to develop his mouthpieces and mutes business. Another student was trying out an instrument, and complained the high C wasn't very good. Denis picked it up and played a dozen immaculate high C's, loud, soft and everything in-between. Maybe the student kidded himself the instrument had been changed. Anyway, he could hardly continue complaining about it!
If you feel that having a top player blow your instrument helps YOU play it better, I'm not going to argue with you.