I tend to anchor my thumb on my 5'ers B string.. I met a musician up at GC the other day and he was showing me some hand exercises.. At one point though he started criticizing me for resting my thumb on the B string.. He said that I SHOULD NEVER REST MY THUMB.. I said, well what about all the basses that actually have thumb rests built on them? He said you should never use those.. I was thinking W.T.eF? But now since meeting that guy I have been really self conscious of where I'm resting my thumb at.. I tried playing with my thumb "floating", but it feels totally awkward and I have no root/pivot to help me know where my fingers are in relation to the strings... It is much more comfortable for me to rest on the B string, or sometimes on the edge of the neck, or a pickup, all depending on where I'm playing at for tone's sake..
I was going to start a thread today asking advice about this, but decided to search first (thank god because I found this thread)..
From what I've read here it looks like there is nothing wrong with resting your thumb.. It actually seems quite common.. I think that musician the other day had some good pointers, but this issue about not EVER resting my thumb was some BS...