At one time, pre biomes, you could actually predict where they would fall. Well, for an entire stand, anyway. I had a huge tree farm (about 500 spruce) and all I had to do was cut one at each end of the stand, and the one which fell away from the others, indicated which end to start at and every single tree fell in that direction. Now, it seems completely random. I tried everything from orienting each sapling that I planted in the same direction to trying to cut it when it swayed a certain way in the breeze. Nothing seems to work. The only thing I can think of would be to build a long earthen mound however long one needed it, and plant the trees along the crest. That way, they would either fall or roll to either side. At least that would keep one from having to root around in thick timber to retrieve logs and saplings before they despawn.
Edit: You know what ? I found the perfect solution to the tree issue. Just plant the damn things further apart. This will only work on your own tree farm, of course, because in the forest, you have to take them as you find them. All you need is a nice big piece of reasonably flat ground.When you plant your saplings, give each about a fifteen to twenty foot radius, depending on the size of the tree. Problem solved.