I've been playing 5-strings for the last ten years or so, which due to the width means I've played with my thumb somewhere on the back of the neck, fingers at right angles to the strings. Have just started playing 4-strings again, and with a very slim-necked Warwick, I find that my thumb wanders round to the side of the neck on occasion. I haven't gone to the extremes that you can see Sting doing though.