Depends. If it's a reading job, it isn't a good use of my valuable time to learn the material, but it helps to be familiar with it. In my country band, we often take the song round again if people are up dancing, but I know how each section goes, and how to link them. And for depping in other bands, I may know how the original recording go, but I frequently have to adapt to a different song structure, or even the wrong chords!