Fretting Hand Thumb Position

[-executor-]

I've heard that the thumb on your fretting hand is supposed to be perpendicular to the strings. When playing more simple things I can keep this positing but when I play more complicated things, my thumb tends to go from perpendicular to parallel and sometimes somewhere in between. Is this bad technique? Should I try to keep my thumb on my fretting hand perpendicular to the strings?
Any tips will be appreciated.
Thanks

[Wes]

I'm having a problem like this, too. With me my thumb never goes parallel just slightly angled. This only happens when I use my little finger on the low end of the neck; because I have to stretch my little finger to get to the 4th assigned fret.

[chaosMK]

The best bassist I've seen tend to keep their LH perpendicular to the strings.

[SLaPiNFuNK]

Behind your Middle Finger (2) on the lower 3rd of the neck...

end of story

like you are holding a soda can, just move that position to your neck...

Ill take a pic when I get home if needed...

[PrimusNut]

keep it "perpendicular" when u can.. it doesnt matter if you veer of from that rule for harder stuff. if it doesnt hurt you and makes you play better, thn its fine for now. just TRY to stay the textbook method whenevrr u can (i never do this lol)