The best way to figure that out is to open up Satistics from the Main Menu, look what each character is best at, and exploit that stat. From my experience, Final Fantasy follows the economic principle of comparative advantage - you should always allocate your resources so that each character can do what they're best at, rather then trying to have well-rounded characters.

One exception in this game is Setzer. From what I remember from my last playthrough, there is no one stat that he particularly excels with, so I made him my red mage - gave him a variety of magic, and raised ATK, MAG, STA, and speed. It kind of works out too with his long silver hair.

So yeah, characters like Mash, Cyan, and Edgar you should max out their strength or stamina, and characters like Relm, Stragos, and Terra you should max out their Magic. Looking at the stats of jobs from I, III, and V also help.