I like this reply a lot. But in reference to Adam Neely's idea of the thumb pointing north, it should be pointed out that his main bass is a 4 string P bass. On a 4er you only have to reach so far with your fingers, so while you can orient your fingers perpendicular to the strings with a narrow neck you also have the option of your fingers pointing more diagonally across the fingerboard. And with that approach the thumb may be more relaxed in a northerly orientation. It also allows the palm to create more leverage and opposition to the fingers. However, with a 6 string bass the neck will be significantly wider and the thumb cannot wrap around the neck the same way as a 4 string. Also you need more reach with the fingers which in many cases will dictate the fingers are perpendicular to the strings and the thumb also perpendicular is more natural to this approach. But as Mushroo pointed out, the key is to use as little thumb pressure as possible which I think is simple wisdom and common sense.