Some guys who use slap ubiquitously raise their bass up and approach it from a sort of underhanded position with their thumb.
Here's Mark King, as an example.
The underhand approach is certainly not the only legitimate way to play, but I felt myself drawn that way pretty early in my development as a slap player and never changed back. I do feel as if that angle allows me better access to all of the strings without the rest of my hand getting in the way.
Here is a clip that illustrates my approach.
The downside, I think, is that one cannot switch from fingerstyle to thumb technique as quickly this way. I actually feel myself shifting the position of the bass to the right when I make the change. On the other hand, Wooten, who plays fairly overhanded, can move from thumb to finger very rapidly. Some of this, no doubt, depends upon the weight distribution of your bass and your body.
That's my input. I'm sure you will find your own preference with time. Best luck.
