It's really a case-by-case basis. When it comes to more "structured" music, I try and nail the bass line as closely as possible (oh how I've wanted to smack some of these modern rock producers in the mouths who bury the bass under kick drum and 17 layers of guitar...makes trying to pick out runs and intervals real fun

). I'm primarily a fingerstyle player, but if a song has a prominent pick sound, I'll use a pick. On the other hand, blues, jazz, and similar styles definitely call for improvisation.
With that said, I play in a single guitar band, so sometimes I have to busy up the bass line under solos to beef up the rythmn section.
And what's everyone got against mixolydian? You gotta play something against those dominant 7th chords.

"He's the electric horseman, you better back off!" - old sKool making a reference to the culturally relevant 1979 film.