I picked up guitar
seriously though, it can't hurt to know a handful of guitar chords, especially if your theory knowledge is decent, you'll be able to communicate a general idea or specific idea much easier that way. Of course, it's equally useful to learn chords on the bass.
that said, I often have a melody in mind, or a certain feeling, and I work from there.
There are a million chords that can fit over any melody, so I usually let the melody drive the song then fit chords under it later. Personally I think that the bass is a great instrument for melodic expression, and as such, I often write melodies with the bass.