Being a good timekeeper is absolutely essential for a bass player. I've had studio sessions where the drummer and I have been recording to a click, but at some points the music has just [i]needed[/i] to speed up a fraction; the drummer felt it before I did, and increased a couple of bpm. I made the decision to follow the drummer rather than the click and we ended up with a great take. We had made artistically interpreted the score and had made music, instead of an audible representation of the dots and lines.
I think it's only once you have developed solid timekeeping as a player that you can start to monkey around with it. And get away with it.