Are 'dirty' flip-flops 'appropo' in the workplace?

Ladies, I know that those of you who work in an office-type environment like to be comfortable, yet fashionable when it comes to your choice of footwear. But is it ever appropriate to bring as a change of shoes, dirty, well-worn, flip-flops and wear them? I'm talking about the ones with the visible 'toeprints?' And ladies, how long do you wear them before the prints start showing?