The head has to align with the shaft so that it doesn't steer the arrow one way or another off center. I stand my arrows on their tip and spin them slowly, looking for the broadhead to "bobble". If it does, I try another head until I find an arrow/head combination that spins true. With hotmelt glue you can heat the insert a bit and reposition the head until it is true. With the newer hidden insert (HIT) arrows you can resquare the shaft until it's right, or you can resquare the end of the insert on a standard shaft. (See the G5 Arrow Squaring Device.) Another way to do it is to put the arrow in something that it can roll consistantly on, either 'X' sticks (or nails, as someone stated), a large open text book, or I use an old arrow straightener, then put the tip of the arrow up to a box with a reference dot touching the tip. As you spin the shaft, the tip of the broadhead should stay centered in the reference dot, if it wobbles you need to correct it. Some heads are more sensitive to this than others ie.) one head may allow you to be a touch off but another may be very sensitive. It also seems that the faster you push them, the more critical they are. (Yet another plus for heavy, slow arrows...)