I have found over the years and for me personally I'm better throwing a bag overhand. I grew up throwing footballs and not softballs and it just more natural to a point. It really comes down to the size of the throwbag and amount of water load. Waist worn, kayak 50' bags, small cord stuff is all overhand. My large rope, long line 70', hanging from my raft bag is underhand. I spent time learning to throw underhand as a guide, but it didn't come naturally for me.
On a side note: It doesn't matter how you throw it, if it's not packed right and prepped before each trip. I have a jacked up knuckle due to a borrowed throwbag at a boat recovery. I was out in the river, on a rock, and clipped a throw bag handed to me from shore. The rope failed to deploy after ten feet, wrapped around my hand, dragging me off the rock, into a river over head deep, stretched tight between a pinned kayak and a kayaker turned upside down trying to fairy a knotted up throwrope to the farside of the river. Pack that shit right yo.
I love to dance, but who needs the music- It throws me off.