For my regulars, I just do the job and don't even discuss it when I'm done. For one time cleanup customers, a lot of the time they are watching and will actually come out with a check before I'm even done. If not, I will usually know and ask them to walk the lawn with me in case I missed anything, not being familiar with their property. Most of the time they don't bother and just write the check.
Have you ever seen what a property looks like after the average homeowner is done doing a leaf cleanup by themselves...it certainly wouldn't meet my standards. So in general, anything above and beyond what they could do themselves is going to get their approval, especially if you give it a final mow with the bagger on and leave a nice set of stripes.