Under ideal circumstances, I feel best about playing covers when I really know the tune. That includes a lot of listening to the original performance, an understanding of the harmony, and a strong sense of the overall dynamic and emotional content. But I've often been called for a wedding or some such gig and been told, day of: "Oh, by the way, we're going to do 'Copacabana' and 'I Wanna Be Your Lover,' so have them ready and I'll see you in an hour."
Under these conditions, expediency beats thoroughness. One cool website I found has several bass transcriptions of a bunch of commonly played tunes that I never bothered to learn. The transcriptions are bare-bones (no chords, and I haven't checked on the forms) Check it out: http://jeremyschmidt.890m.com/public_html/Transcriptions/