In my current playthrough, here is what I have...

Bartz- Monk (has also trained in Thief)
Lenna- Summoner (has also trained in Black Mage)
Galuf- Berserker (has also trained in Monk)
Faris- Ranger (has also trained in Knight, Ninja, and Mystic Knight)

Now for the why. What I do when I first start the game is right down the stats of each character's Strength, Magic, Agility, and Stamina. This is while they are Freelancers. These stats do not change until you master a class. Also, I write these stats without weapons and equipment on the character's, to get the bare-bones stats. Then I noticed that Bartz's highest stat is strength, Lenna's is magic, Galuf's is Stamina, and Faris has a tie between Strength and Agility.

Now, I choose Jobs that boost their higher stats. That's why after the Wind Crystal broke; the Jobs were Thief (Bartz), Black Mage (Lenna), Monk (Galuf), and Knight (Faris). When the Water Crystal shattered, it became Monk (Bartz), Summoner (Lenna), Berserker (Galuf), and Ninja (Faris). When the Fire Crystal broke; Bartz, Lenna, and Galuf stayed the same, and Faris Became a Ninja. Finally, when the shards came out of the Chocobo, Faris then became a Ranger.

When the characters master a class, I will choose the next suitable Job for them to complete. That's how I like to do things.