From the reactions in this thread, i suppose, it all depends on the default position of the hand. If someone uses the "typing" position, ASDFspacebar, it's understandable that he uses thumb just for the spacebar. If someone uses some kind of "gaming" hand position, such as shiftAWDC for FPS games or shiftASDB in my case, it's quite convenient to use the thumb for lower line keys in addition to the spacebar.

However, when you start playing, you usually don't think about the ideal place for your hand. You just go for the position, that seems natural and you are confortable with. Therefore I wouldn't bother asking, whether you are allowed to have your hand placed like that, and I would just stick with what feels natural for me and play more to improve the precision over time.