I might house-rule that iron weapons are default, and that steel weapons get +1 to hit and damage.
Soldiers in any well-equipped army carry steel weapons; Roman soldiers (at least some of them, that is) carried weapons made with Noric steel. (Ovid Metamorphoses, 14.712: "...durior [...] ferro quod noricus excoquit ignis...")
But ill-equipped goblin tribal warriors might carry weapons made with lesser (non-steel) iron, and a farmer's knife might be a +0 dagger, and so forth, thus making +0 common among non-military-grade weapons, +1 common among military-grade weapons, and +a 2 sword is either masterwork steel, or "average" steel which was then given a +1 enchantment.
Of course, taking the time and trouble to lay a +1 enchantment on an iron or low-grade-steel sword is kinda silly, when you could instead start with a masterwork steel weapon and THEN enchant it.