I like Victor Wootens technique. Its all just another way of hitting the string... He practices being able to play on all strings with either slapping with the thumb (in both directions: up and down strokes) and with all four fingers in every combination. Dont restrict yourself in any way!
Hes super human though... in typical situations I will hit the E and A strings with my thumb and mostly pop the D and G strings.
On a side note... Marcus Miller does an awful lot of playing with just his thumb.
