Tier 2: Down
Tier 3: Up
Tier 4: I went with up because I use it as an escape. If you use it for damage go down.
Tier 5: Up seems like a better choice but I have found you have to be extremely close to teammates, but the down skill doesn't seem all that useful
Tier 6: Down, the snare is just a speed reduction. I'd rather be tankier. Although, the speed debuff could help if you're chasing someone down.
Tier 7: Depends on if you use Burning Blood for damage (up) or as a snare (down). I went with up but I think I'm going to respec and go for down.
Tier 8: Up
Tier 9: Could go either way. A lot of people use up, but I would use down, as you can get back in the fight quicker, which is very helpful if the enemy is attacking a tower.
Tier 10: Really either one. I like the snare better though because it indirectly causes more damage.