It seems that the visuals are used for the arrows, so if your archer is standing behind a tree, the arrow hits the tree and stops. The guy next to him might be able to have an uninterrupted shot, however. Incoming arrows are on the arc, so they are likely to hit the larger "branches" part of the sprite, so you will get some cover, at the cost of some wasted arrows. If you archers are hiding deep in the forest, then they can't effectively shoot at anybody. Here's a case where the arrows aren't mathmematical abstracts, but it actually tracks the arrow sprite in its arc, and if it hits something, then there's your shot!

I agree with Yagyu Jubei: have the archers at the edge of the forest with yaris and no-dachi ready to charge out from under cover. This is especially important to do when fighting against cavalry!

-- B)