In the case of avoiding the windpipe, it's optimal, but not necessary. the arteries are right under their jaw, earlobe area. I usually only do one side because I end up cutting myself doing both. Generally I dont hit the windpipe, just because it's an odd angle to get both.I've worried because I've seen instructions to cut the arteries on both sides but avoid the windpipe, and I can't figure out how they do that.
It did take a while to find a way of doing it that I felt comfortable with. I tried several different ways. I guess the biggest thing to keep in mind is, even if you do screw up. Get it done as quickly as you can, like everything it takes practice, and mistakes are a part of the learning process. Remember that once they're dead, they are dead, and not feeling pain.