I've had the crowd request Mustang Sally sooooo often, it would be foolish not to do it, IMO. Freebird, on the other hand - the reason I wouldn't do Freebird is because it's long, it's not a particularly good dance song, and it's really difficult to do well, so it ends up sucking. Otherwise, I would play it, too, if requested.
I think if you are in a cover band and your goal is to play regularly, you should be willing to play almost any song that the crowd wants, that you are able to pull off.
For heaven's sake, I was at a gig Monday night and we played "Mr. Sandman" totally out of the blue. The crowd loved it! Who would have thought?