That seems to be what travel writer Christopher Elliott is advocating in his USA Today column...
We're well aware of the technique of reserving the smallest and cheapest car and hoping the location is out of them and has to upgrade you. As he writes elsewhere, however, you'd better be prepared to be stuck with the category of car you've reserved.
There may be one or two useful tips in this column, but it's a little bit reminiscent of some of the naive suggestions offered to score an upgrade to First Class on your flight: Dress up, tell the gate agent it's your anniversary, bring a box of candy, etc.
Come to think of it, Kathy's brother Tom did give a box of candy to South African gate agent at IAD and we managed to get exit row seats on that ghastly long flight from Washington Dulles to Johannesburg. Of course, we had Star Alliance Gold status and were first in line so we still don't know how much good the candy did.
Generally speaking, front line employees with even a little experience will have experienced enough flattery and schmoozing to be immune. Probably the single best suggestion in the column is to be "friendly and polite," advice we hopefully don't need.