Here's my opinion on slap bass left hand technique.
The much maligned (here on TB) "baseball bat", thumb wrapped around the neck, left hand grip you see many Rockabilly players using is actually PART of the slap bass technique many players use. The thumb is commonly used to mute the open E and sometimes the A string. To keep them from ringing constantly while slapping.
When playing slap bass, the whole hand is sometimes used to slap the fingerboard and strings, not just the fingers. This is why the left hand thumb is often used to mute open strings. Having open strings ringing away in the background is NOT acceptable in slap bass playing, IMO, and sounds like crapola.
A few players avoid unwanted ringing of open strings using left hand muting alone, such as Barre-chord style muting with the index. Some guys use oddball fingerings, such as curving over your pinky and ring finger to damp the E string (

) Watch Jack Hanlons Youtube demo video to see this style, starting at 1:36 during the root-fifth syle bassline. he rarely wraps his thumb to damp the E string. Here:
http://www.youtube.com/watch?v=lwu32hkI9C0
Many great slap players use the "thumb wrapping" left hand technique, such as one of my favorite players, Pete Turland. He shifts his whole hand often to "fret" the notes and does this in an incredibly smooth way. This is a more common and "old-school" way to play slap bass. Great example here:
http://www.youtube.com/watch?v=u0iicnBBy8k
Many great players wrapped their thumb around, including Willie Dixon, Pops Foster, and lots more old-time bass players who mostly played on low tension gut strings
Please keep in mind that this left hand technique could cause injury if attempted using many of today's high-tension steel strings. Slap bass requires the use of low tension strings, IMO. I am not advocating throwing proper technique out the window, and I believe beginners should learn at least proper pizzicato left and right hand technique before getting into slapping. The meat and potatos of my practicing is still mostly pizz, with slap being the dessert.
Please use your best judgement, if what you are doing causes pain - stop and find a better way or ask a teacher.
Also, learn to walk before you try to run.
Just my 3 cents.