For me, it really depends what the song is and what it's being used for.
If it's a simple tune then I'll just learn it by ear and not stress about it.
If it's something where the bass line is distinctive then I'll break it down into logical sections and learn it note by note.
If it's for a recording, then I'm generally given the freedom to write an appropriate bass line which I'll keep notes of in case I happen to forget it - it's never happened yet, but you never know.
I don't bother trying to download tab for new material any more, it rarely resembles what it's purporting to be a representation of.