I think you mean max level, not minlevel, since min means minimum, and you wouldn't start "Tiering" it with a minimum level until third tier level 150 >P (which is why I think you mean maxlevel, as in the equipment's runic maximum level)
Otherwise... it doesn't sound like a -horrible- idea. How would you handle the bonus stats? Since, obviously, you'd either have to be paying for them as you go so that you have them at maximum tier, or else it would suddenly become a blank custom?
While the AC boost from its runic properties is nice and helps... part of the reason -for- customs is because of the bonus stats you can get from them. Including health, move, mana, and resists. As well as the hit, dam, and AC.