Do cashiers/waitresses/salesgirls flirt just to kiss up to the customer?

I'd like to hear some honest answers, preferably from girls who do those jobs. I often get girls flirting with me in public, but they are almost always doing a service job of some sort at the time, which leads me to believe that they are just coming on to me for a better tip, employee review or whatever. Or, are they so sick of doing their job that they hope to latch onto the first guy who comes in with expendable income and no wedding ring? Am I right about any of this, or am I just being too cynical and paranoid? Since a good 95% of the girls who flirt with me are working at the time, I can't help but be suspicious... -_-' Please, let me know. Thanks! :)