If each party in a transaction over-delivered by 5%, what room would be left for disagreement?...
We can't completely control how a customer treats us, but we can do our part and go a little beyond what was agreed upon initially. Especially at the end of the job. It will be noticed, and you will benefit one way or another (tip, concessions, referrals, conflict avoidance, etc). A lot of transactions leave people feeling like they got exactly what they bargained for, or that they were taken advantage of. Whenever we feel like we got a great deal, not only to we take notice, we tell others about it.
The same principle applies to employees and sub-contractors. In fact, if you find yourself frequently over-delivering and it is not reciprocated, it may be an indication that it isn't the right fit long term. In this way the 5% principle can also act as filter for finding the best people to work with.