OK, talk to you later then! - My access will be iffy once ecurbh wakes up, but I will be back sometime!
My experience with tie-downs is real limited. I've never used one. I have seen horses that have had them used for long periods, and they do tend to develop 'upside-down' necks... all the muscling on the underside of the neck from leaning on it, and no muscling on the topline of the neck. I think that happens most when it is used instead of training to hold down a "too-high" head ~all the time~.
I have used both running martingales and german reins/draw reins at times to correct evasive head behavior.