I think its more a stylistic thing than anything else.
Some people wrap their fingers around the sides of the neck and hold their opponent that way, while others cup the back of the neck with one hand and cup their own hand with the other to hold their opponent.
If you are able to control your opponent and land effective strikes with your hands on the neck, why change what works? But if you are having issues landing effectively, I would recommend either going with the cupping style, or just ask your instructor to assist you.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. Thatís it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit