The situation you described is not a standard type of transaction like table service at a restaurant. If I'm the guy behind the machine and not an owner/stringer of a shop, any gratuity would be a nice bonus for that extra quick, reliable turnaround. A better and more appreciated gesture might be to match the service fee from time to time, obviously less frequently to make it more impactful while still not jacking your cost up too badly.
When stringing at pro events, I have often received a little bonus but the best extras I received were from guys leaving for the next tour stop who would give a few sticks to do on their way out of town. That's a win/win for them and me!
"When a man is tired of London, he is tired of life"...Samuel Johnson 1777
"When a man is tired of Federer, he is tired of tennis"...stringertom 2014