We're a fairly democratic bunch. The main song writing force in the band is the guitarist, and tends to develop ideas by recording demos and giving them to us. Sometimes this demo can be fairly fully realised with some bass ideas in there already or it could be as simple as strummed chords. If there's some bass there then I'll use it as a starting point, but will put my own spin on it. If not then I'm pretty much free to come up with whatever. We perfect it at practice while the singer hums and jots down ideas then she goes off and writes lyrics.
Speaking for myself, I'm only starting this bass playing and being in a band caper, so even though I wouldn't anyway I am in no position to be arrogant about suggestions/criticism. I'm happy to take on board suggestions. I would never claim to like it but I accept when I'm wrong as an opportunity to make something right and learn something. My trying not to be a knob is probably one of the reasons that the band have put up with my lack of knowledge and experience