i am a self taught slapper, and ive noticed from watching videos on youtube etc that most bass players slap with their thumb pointing up or paralel to the string, but i slap with my thumb pointing down. is this one of those nasty habits that will hinder me from playing faster later? or is it just a preferance? also i have a bridgecover for my Standered fender jazz (mexican) and im wondering if theres anything i sould know about installing it other than drilling two holes in my bass, and if anyone has expeirienced troubles with it covering the bottom pickup? thnx

I would never tell anyone that how they do something is wrong when it comes to technique. What I will say is that I do have an opinion based on my personal experience as a player/teacher for many years. In my opinion pointing your thumb down for slap puts your fingers in an bad position for popping. Since the traditional technique is based upon your wrist movement it is a more advantageous to have your thumb pointing up or parallel (my personal preference).
As for the cover, I've never installed one so I cannot comment.

hmm ok thank you ill keep that in mind

I too am a self taught slapper. I started out with my thumb pointing down and couldn't figure out why I was having so much trouble learning to double thumb. Then I heard Anthony Wellington mention thumbing parallel to the string (in Vic's Groove Workshop DVD) I tried that and almost immediatly the double thumbing happened. So I think you have a lot more rhythmic option when thumping parallel.

I don't understand what you mean by pointing your thumb down. I've been doing it parallel since I started, so could someone explain/show how you would do it with your thumb pointing down?

http://www.youtube.com/watch?v=FRAdKS8hmxA

watch starting at like, 1:15 in the video.