The only real answer is it depends. I was in a tribute band once and we came from all over so we never rehearsed. It was vital everyone learn and play every part note for note as per the agreed versions. You could have improvised but then how do you know one of the other 8 musicians wasn't taking a cue from the part you changed? However in most other cases it isn't at all important to learn the exact part. Some bands I'm in I improvise everything, never the same twice and no one gives a hoot. In another the band leader is crazy anal about everyone playing precisely what was played on the chart single version, not an album or live version, god forbid. I tried asking if they thought the original artists themselves ever played it like that live but to no avail. Other bands I'm in I do as people here suggest and only recreate those stand out bass parts without which the song would sound unrehearsed. Another band I'm in play almost all originals so they're my bass lines and I can do whatever I want with them. Of course there are covers where I love the bass line and want to learn it for that reason alone, and others where the original was written by someone like James Jamerson and I think to myself if it was good enough for the master who the hell am I to change it?
Like I said, it depends.