I found that if you have the belt of knives, you want to pack your arrows as tightly as possible, and pick up as many as you can. The stacking bonus from the knives with 9 tightly packed arrows (3 horizontally along the bottom, then 6 vertically above them, bow to the left) vastly outweighs any bonus you will get from the arrows having empty space in front of them. Also, your arrows only get bonuses from space in front of them, rather than from all sides. (unless that bow changes that?)