Growth tier is about level limits and what titles you can assign to a unit. Units with lower growth tiers are more front-loaded, that is to say, they don't grow as much, and have lower limits on the strength of the titles you can put on them, but you get more of their Ooomph value from the moment you recruit them.
In other words, D Tier and the like units can be decent in the early game or the 1st playthrough and whatnot, when you don't have many levels and don't have many titles unlocked. But for the really good formations, you want the S tier units, and you want to recruit them under optimal conditions with optimal titles. Though it's fine to recruit them initially to level them and raise loyalty and whatnot, and then re-recruit them later to improve their titles.
Commander units will be S tier if that's the highest Frontier had, or SS tier if it's like latter games. Don't remember.
Generally, by the time you really have your forces going and whatnot, the majority of your forces will be S Tier, with some A tier and the rare B and C Tier popping up as well. I don't remember enough about Frontier's unit set-up to say for sure. I also don't remember the level limit in Frontier, but if it follows other games, the S tier units can reach ~150, while the A tier would be 140, then B Tier 130, so on and so forth.
That said, also take into account the relative strength of a unit too. A Goblin might have S tier growth, but its skills and stats are still shit and no amount of titles are gonna fix that.
