Personally, I grow both, and trim neither.. I think they are both equally important. Numbers 6 does not say grow the hair of the top of your head, and trim the beard... It says to grow the locks of the hair on your head... The face is the front of the head, so to me, this means... locks all around. from the back of the neck to the face, we should not trim.
It is not esential to be Rasta.. but more of an "extra vow" I don't think Jah would punish one for not growing locks, but if one takes the vow, he better not turn back. You can not break contract with The Almighty.
Each Rastaman trods his own trod, though to the same destination. Some Ras drink, some do not... some eat certain meats, some do not... get me? Each Rasta, in my humble opinion, makes his own vows with Jah, and it is personal between them.
I hope this is helpful to the I
Selassie I Bless