For the most damage on the outside of the leg absolutely angle down. But it does slow my kicks down too, so if it means the differense between checking the kick and not having the kick checked, don't angle it, just get it there quick. Inside leg kicks I've learned straight accross or slightly upward is best. On a side note, someone said to aim 1/2 between the knee and hip. I was taught to aim only 4-6 inches above the knee. I'm not saying he is wrong, but there might be different schools of thought on that point.