I really can't agree with the concept here.
If we feel obligated to give 15-20% to a waiter we may never see again because he filled my order and water glass, how much more that teacher of my kids, or preacher to my soul!
Whether we like it or not, the restaurant business generally has a waitstaff compensation system of tipping. The only one I know personally who has revealed her wages and tips makes $2.13/hour + tips, of which she must give 3% to the bussers. If a church staff is in any comparable situation, it must be a small church of poor people, where the pastor's salary is $100 to $150 per week, so he must rely on tips for a living, or else have outside business interests which may require a lot of involvement. So instead, a church should see to it that their ministers are paid well to the most reasonable extent, and then personal gratuities should be from personal services (weddings, funerals...).
A second thought is that while most of us (hopefully) do appreciate the ministers who do much for us, where does this stop? Tip the piano player? the church janitor? your child's public school teacher? the police officer who walks or drives by your house each day? the city engineer for improved sewer operation? Some of these-- the police officers, anyway-- it would probably be illegal for them to accept gratuities, because they have an obligation to treat people equally. This concept should apply to churches, as well. It's not true that everyone who offers 'something extra' to a minister is looking for special favors, but the idea does carry that suggestion. If Fundamental Baptists are so concerned about appearances, how about in this matter?