I agree with ALMOST everything posted yesterday (above) by LostHills, with one exception:

And don't ask me if I "know any John Denver." I'm not a juke box, I'm an artist and I'm here to do my art.

I never minded requests ~ it means that someone is not only listening, but is considering staying around to listen some more! Such patrons deserve consideration, not only as friendly fellow humans but also for the income they are likely to generate, both out of their own pockets and from the additional listeners/lingerers that they are likely to encourage.

I always made it a point to have one or two songs in my repertoire that could be appropriate responses to just about any request. For example: "Gee, I don't know that one, but since you like country music, how 'bout this old Hank Williams classic?" or, "Sorry, I haven't learned that one, but here's a song I know that was recorded by the Beatles/Elvis/Doc Watson [or whoever]."

Now, I understand that this does not apply if you're a songwriter interested only in performing your own songs, which may be admirable but is probably not the most lucrative approach to take. You might consider the strategy of Glen Hansard's character in the film "Once" ~ perform recognizable favorites ("established songs") during the busiest hours of the day, and sing your own stuff during slower periods (after dark, perhaps) when the listeners may be fewer but those who are in the area might be more likely to stop for a while and listen more carefully.