I always tip repair guys (yes I pay for repairs such as tips at tournies & such). I did it before I ever built cues. I believe it's customary, but not particularly mandatory. I relate it to my waiter/waitress whom I tip for good service. I don't have to but if they do a good job then the tip shows them I appreciate it AND encourages them to work hard for me again next time.
I also tip the house man &/or tournament director whenever I cash in tournaments or money matches. Lots of players think i'm weird for doing that, but they don't consider that I get special perks for that generosity. When a player comes to the room looking for somebody to gamble with, the room immediately calls me. When i'm playing a tourney & want to go somewhere & sit down for a lunch, they'll hold my next game til I get back & am ready to play. So long as I don't take advantage of them, they take extra special care of me. Scratch my back & i'll scratch yours. Same with cue repair.
It's a common courtesy kind of thing. It's called gratuity, which means a way of expressing gratitude. A thank you that means more than just a smile & a nod. The repair guys can use the extra cash and they'll be sure to take extra care of you if you tip well. Of course you'll get what you are paying for and the job will be professional. But once the repair guy learns that you tip well, you'll most likely be getting much more than you pay for. Instead of just a tip, you might get a shaft clean with it, etc. for the price of the tip. Gratuity is a two lane street. You show gratuity & it'll be expressed back to you. That's how I feel, anyway.