The conventional wisdom is that because the blades do no extend (which would effect their flight) until impact, you do not need to tune your bow to them provided you've sighted/tuned to the same arrow (weight/spine). As a practical matter, as long as you shoot into a target which won't damage your mechanical broadheads (cardboard into sand, etc . . .) I'd always confirm point of impact with the arrow with which you are going to hunt. Better to be surprised in your back yard than in the woods.
YES! You need to tune your bow to work with your arrow/tip combination regardless if it's a field point, expandable or fixed blade head. You owe it to the animals you're hunting to be able to make a quick, clean, ethical kill. In order to do this you need to make sure your arrow is flying true and you can make the shot count. While mechanical broadheads should fly very close to your field points, that isn't always the case. Taking the time to practice with them and tune your setup so they're flying perfectly is time well spent.
