I've used mechanicals (and fixed blades if it matters) for quite a while. I've had bad arrow flight and impact points that were off from my fieldpoints (especially with Spitfires for some reason) when using mechs. Was it a tuning issue....probably. Even though I'm much more educated now when it comes to tuning, I like to make sure my setup is doing exactly what it is supposed to do. The only way to do this (in my opinion) is to shoot the broadheads.
While I am VERY grateful for the advice I've received from this website, I think it is TERRIBLE advice to tell someone that they don't have to shoot their setup with a broadhead (of any type). There are too many things that can go wrong to give you bad broadhead flight, even with mechanicals. Shooting the heads will point out these flaws in your setup. Please don't take that the wrong way, I'm not trying to be an A**hole. I just think that is bad advice to give, especially for those on the sight that might be new to bowhunting and reading these threads..
On Spitfires, I'm not bashing them. When I used them, I thought they were a great head. I got great blood trails and received great performance from them. I stopped using them due to cost and durability issues with the ferrules. I had too many heads that had blade slots collapse just enough to make the blade deployment too tight for my liking. It was getting expensive to keep replacing them. I can buy and maintain Steelheads for much less $$$.
Thanks to everyone for their advice. I'll look into dental floss or fishing line to tie the head closed. Does anyone else use a method different those listed?