When you go to a restaurant, and you're considering how much of your hard-earned cash to spend as a tip for your server, I just want you to consider two things:
First: when you have a server who is inattentive, or distracted, or clumsy, remember that such were some of you. Was there any point in your day when you were brusque with somebody who needed your help? Have you ever forgotten an important task, or a minor one? In that moment, did you wish for mercy? You have a chance to be an instrument of that same mercy to your server. Servers know when we've* done a bad job. We are, frequently, painfully aware of the times when we have given lousy service. A bad tip makes us feel justified: the customer deserved the bad service because there wasn't a bigger tip.
On the other hand, a good tip for crummy service is like heaping coals of fire on the head of your server. That's an analogy, by the way, not to torture but to sacrifice; by heaping coals on someone's head, you allow them to pass through the fire and ascend to God like the smoke from the altar. The church fathers were fond of speaking of our prayers ascending like smoke, and I can tell you from personal experience that I was moved to pray more by the generous tips for mediocre service than by the reasonable tips for reasonable service.
In other words, do not return evil for evil, but rather repay evil with blessing.
Second, and even more importantly, what do you have that you did not receive? And if you did receive it, why do you boast as though you did not? (1 Corinthians 4:7)
It is possible, from a certain perspective, to talk about having earned or deserved your material blessings. If you are a fine carpenter, the best in the shop, and you work long hours producing excellent work, then you have "earned" your wages. But why are you working at this shop? How did you get your training? Who trained you when you were a lazy five-year-old and taught you diligence? That was all grace. Your truck that keeps taking you to work long after you thought it should be dead? Grace. The fact that your boss is a just man, who pays you what you are worth? Grace upon grace.
Stinginess denies that grace. Freely you have received; freely give. Do not resent the fact that there is a tip line on your check. Look on it as a chance to imitate the generous grace of God by bestowing on somebody a portion of the grace that you have been given.
*I'm not a server now, but I worked as a waitress and prep cook in restaurants after I graduated high school and all through college.