Totally agree with Visa .. where do you rest your thumb anyway? .. i usually employ a floating technique .. meaning my right hand is movable, sometimes i dont' rest on anything and i just play like that, it's years of practice lah .. =) .. either than that i'd rather rest on the pickups or my low B string (i play a 5er) ..
i typically rest on the pickup for the E and A strings, and the E string for the D and G strings.
Occasionally I might rest on the A string if I'm playing a string of notes on the D and G.
If I'm playing something that's chugging heavy, chunky E string notes (songs like say, Enter Sandman) I sometimes rest my thumb/hand at the top of the body (yeah... I have big hands) because it lets me really thud the notes out. This is even rarer though.