Actually, you don't have to be as good as Alex and Geddy to make it work for you. Whatever you find comfortable to do is how you do it. It's all good!
I'd be the last one to tell someone else how to play; I don't want to be the Technique Police. It's not all good and the only reason I argue this point at all is that I'd hate to see some beginning player look at Geddy Lee and think that if it's good enough for Geddy, it's good enough for him. People that play well, but have bad technique, play well in spite of bad technique, not because of it.
It's really just a matter of physics. If your thumb is behind the neck you have a longer reach. If you have a longer reach, you don't have to shift as far or as often. Less shifting leads to a smoother sound. Less shifting allows you to play faster with less effort. This is not to say that there is NEVER a reason to wrap your thumb, but for the vast majority of situations, the thumb behind the neck is preferable.
If you are as gifted as Geddy Lee, immune from the laws of physics, or just plain don't care, then wrap your thumb with abandon!
