You're going to shred a target whether you've got the blades tied down or not. I watched a guy last season tear a Block to pieces over the course of a week and he had his blades tied down. Really, I think tying down the blades is tougher on the target than letting the blades open, because the closed blades tear the foam instead of cutting cleanly. I'd second the motion of just buying a set of replacement blades and letting the heads do what they're meant to do while practicing.