I used to do the bounce thing until I watched some of Vic Wooten's instructional video. I tried it his way (slap "through" the string) and since I started doing that, my slap technique improved dramatically.
What specifically improved, in my case, was my ability to slap on 5 or 6 string basses with narrower string spacing. The bouncing technique was never a problem on 4-strings or wide-spaced 5 strings, but when I was playing Spector 5-strings, with 17mm spacing, my slap technique felt and sounded sloppy. Using the slap "through" technique allowed me to play much cleaner on those basses, which was great because I loved the tight spacing for fingerstyle and now, I feel confident slapping with the tight spacing as well.
What I also found was that the slap "through" technique yielded MUCH cleaner results when recording. Using the bouncing technique, I would hit the strings way too hard and I really had to struggle with that technique not to pound the strings so hard that it clipped all the time, plus I needed lots of compression to get my slapping down to the same level as my fingerstyle playing. I used to see all the videos of Flea and Louis Johnson and, wanting to sound like them, I hit the strings just as hard as them, but the results I was getting were much sloppier. I probably wasn't doing it (the bouncing technique) the correct way.But, after switching to slapping through the strings, I got much better results when recording, so my bass didn't have to be as compressed and I could run a hotter signal to the board without clipping. Of course, if I had never used a bass with tighter string spacing, I would probably still be using the bouncing thumb technique.
Bottom line is, use the way that works best for you. If you get good results bouncing your thumb off of the strings, stick with it. It certainly works for a lot of bassists who are far better than I am. But, if you aren't getting good results that way, try slapping through the strings and see if that works better for you. Or, come up with your own different way that gets you the results you are after.