How important is hand/finger position in fiber crafts?
The finger positioning—actually the way the yarn loops around the fingers—controls the overall tension of the yarn. Think of it like the path the thread takes on a sewing machine—all those ups, downs and arounds between the thread spool and the needle to make a nice even stitch on the fabric. 

Fortunately the yarn path around the fingers is much more forgiving than the thread path on a sewing machine—get that wrong and…yikes, thread ball and swear words! If you find a finger/yarn position that you're comfortable with while you're learnings knit or crochet, go for it!

When I was learning to knit and crochet the finger positions never made any sense to me either, I just kept getting all tangled up! My solution was to hold the yarn out of the way between my index and middle fingers until I learned to get all the loops in the right place. Not very elegant but it worked for me because I had some tension control as the yarn ran between two fingers. Eventually I naturally settled into the "proper" finger position as my stitch technique improved.