There certainly is no "right or wrong way" and it sounds like you're doin fine. I personally agree with victor wooten and others that the thumb is just another way to hit the string. It shouldn't define you're playing or style of playing.
I use my thumb to pluck when playing chords especially 10ths
I also use it to slap, but not necessarily just in funk style tunes. A valuable lesson for me was victor's method of going through the string rather than bouncing off, which leaves you the option of immediately picking back up through the string.
I have since been perfecting this technique as an alternative to alternate finger picking in for example, eighth note rock or pop grooves. I get an economy of motion coupled with a nice controllable "edge" to the sound.
JKT
