I have been noticing that when I play, my thumb is not straight on the back of the neck, it is at an angle. I'm not grabbing the neck or anything, but is it terribly bad for my technique to have the thumb off at an angle?
It doesnt matter at all what the thumb is doing as long as your hitting the notes correctly. When i play my thumb does different thing depending where on the neck im playing or what strings im hittng. Dont worry about the thumb just as long as it doesnt get in the way
The thumb should be an anchoring point for your hand. Thumb technique is extremely important- none of this guitarist's thumb over the fretboard thing. Which you aren't doing, judging by your post, so that's fine. Angle of the thumb is more more important when playing fretless than a fretted bass (a straighter thumb generally means a straighter hand, meaning you're much more accurate, which is the name of the game when it comes to fretless).