I try to listen to what the other players are doing around me and play something that will compliment the song. The amount of notes and the speed they are played are irrellevant to me,it just depends on the song and the parts that the other players are playing. Of course I love the opportunity to stretch out a little but I must admit that I've played some busy basslines that I would have like to have not played when I thought about it later.
I also try to consider the instrumentation in the band. If I am playing in a three piece with just a guitar,bass and drums I will tend to play more because there is so much space in the music and if I am playing in a six piece R&B group I will play things a bit differently so as not to step on the other players toes.