i currently play a fretless jazz bass (i don't like it, i don't want it, but its all i've got). the action is crazy high due to fender putting the heaviest flatwounds they had on it, and that warping the neck. now i've put nylons on it. this means i've had to be creative (and playing a well setup bass has become a rare relief.)
like ray my thumb goes everywhere: front pickup, bridge pickup, side of neck at joint, side of neck at twelvth fret. i also use my thumb to slap (popping is very hard on this bass), so sometimes i'm just out there. finally, i've started trying a hooked finger approach. sortuv like popping, but i'm just plucking the string that way (i've heard that jamerson did something like this). i haven't figured out where to put my thumb for that one yet. so i'm just everywhere.
matt