I agree a "true Christian" would leave a generous tip to give a favorable image to a waiter, but that is so far down the radar for most churches, it's not even funny. Most people in the pews today are fornicating, committing adultery, stealing software, cheating people in business, and almost any other sin you can think of off the top of your head. Now we can't even figure out if homosexuality is really forbidden in Scripture and is a Baby a blessing or curse from God.
If you are trying to get someone to tip better, maybe they should hear 20 more messages on tithing, cause they don't do that either. If they would steal from God, why would a waiter expect anything else?
My burden has always been for the church even more than the lost. If the church is healthy, the lost will get saved. We can't even get so called Christians to read and study their Bibles. Most are relying of wives tales to build some facsimile of the Christian life and have NO IDEA what Scripture says about any subject. If someone claims Christ's name, at some point they may have to actually read the Bible.
IMO, not all that many people have been lost because a pew warmer didn't leave a generous tip. If you seek more info from them, it's usually a bad encounter from someone closer to them that soured them. The bad tipper may have added to their opinion, but I can't believe that would be the sole cause. If the food was fast and good, would you never return because of a rude waiter?
BTW, my wife and daughter were both waitresses at one point. They have the same war stories about "Christians". They did their jobs as it were to Christ, were humble and courteous, and were well compensated for their labor. A table of 15 Christians was often better than 15 athletes after a high school game. As many tables that stiffed her, their were even more that paid well. At the end of the week she was well paid for her work. It was just as common for the other wait staff to steal from her jar as a Christian table to tip poorly. God made sure she was compensated well in the end. You can't force someone to do something they don't want to do just because they go to church. A pastor could preach on tipping and representing the name of Christ every Sunday for a year, but if you aren't saved, you can't hear any more than the person that won't tithe.
"Lord, when saw we Thee....?"