Paypal popularized the concept of feedback ratings when it introduced them as a means of quality control on it's auction services. It's a very handy mechanism for getting an idea of whether the person you're buying from or selling to has a history of keeping their word.
The same concept can be applied to your daily transactions and associations. Consider your reputation to be your "feedback rating". A good feedback rating can greatly enhance the value of your more tangible marketing efforts. Each person you associate with has a general image of you in their mind. Is it a good one? Keep in mind that every action you take, or don't take, everything you say, everything you do, is an opportunity to add to, or subtract from your feedback rating. It may not be written down, or publicly posted, but it is very real. How will the customer or supplier you're dealing with right now respond when asked "What do think of so and so's?".
Word of mouth can be very powerful, both postively and negatively. You can't control what people say about you and your business or who they say it to. You can control their perception of you by how you interact with them.
You don't know who people deal with or talk to during the course of their day. Your lowest income, lowest spending customer is potentially just as valuable as your biggest spender. Don't cherry-pick who you're going to do right by. Do right by everybody, and accept no less from them toward you. Your efforts and awareness will pay for themselves many times over in the long run.