Depends entirely on the interaction. If I'm paying for work they have a set rate for that is advertised, they charge me that rate and do an acceptable job in their normal workflow (i.e. not a rush) -- I don't worry about tipping. So usually no tip for a base grind, a base weld, etc.
If they don't charge me at all, give me extra useful advice, or charge me well below what they might be able to charge, then some sort of tip.
I work mostly with staff at my local hill -- both bootfitters and ski repair. Works out nicely that the bar there allows you to pay it forward -- so I can just tell the tech to check the board at the Rat after work.