The modern restomodders seems to like the slighly lowered look with 17" wheels. My preference is stock height rear with slighly lowered front. To each, his own. My combo is this: rear - standard eye 4.5 leafs (M+) 24550R16 tires, front - shelby drop with 600# (scott drake) springs at stock height, 22550R16 tires.
The front was high last weekend after I did the spring, shock and new UCA swap, but is has settled down just over .5" so far and I'm probably fine with it. The rear is perfect height and stiffness for me. Comfort is more important to me than stiffness most of the time. I like the overall look especially with the 50 series tires, kind of retro hotrod. I did not want to lower the rear at all because the height varies with a full tank of gas and if I had passengers in the back seat, rubbing might be an issue. My other factor is my wheels with 4.5" backspacing, the tires are close to the fenderwell lips which I did not want to cut or modify. 4.75" seems to be the ideal backspacing for the 67 but of course my wheels are not offered with that backspacing.
Basically comes down to personal preference but you have to consider your overall plan with your car and accessories like brake rotor size, wheeels/tires (backspacing is critical when you push the size/lowering envelope) combo, comfort, performance suspension or combo (4.5 leafs).
It's all about what you want. Thankfully with our cars, aftermarket parts and a little labor the Mustang can be built to any specification or preference.