Finally, here's my supreme guide for correct Team Building in Dragon Quest IX. It took me a lot of planning, but I finally made one of these. Here I go.
DQIX Team Building Guide by Dana412
Part 1: Members
At Stornway's Inn, you can select the number of members you would like to have in your team, just by talking to Patty. It is recommended that your first team consists in:
Warrior: Good damage dealers with high attack/defence. Later, when you unlock the Gladiator vocation, you can change them to Gladiators. Also, high HP.
Priest: Good healers, lots of healing spells and high Magical Mending. If you unlock the Sage vocation you can switch them to Sages and level them to lv 45 for learning Kazing. Pretty high MP.
The 4th member can be anyone you want. In this case I choosed a Mage. Why? Good spell gallery, high MP, Oomph...
Part 2: Skill Point Allocation
Might this is the longest part in this Guide. You should be able to allow correctly the Skill Points won by every teammate.
For example, if you want a Mage with high defence, switch him/her to Paladin and allow 100 SP to Virtue. He/she will gain lots of Resilience and +80 HP.
Basically SP allocation: Any weapon + shield/vocation's special ability.
Proffesional SP allocation: 100 Sword for DDs (this will make them able to use Falcon Slash with Falcon Blade/UFB), 100 Spear/Axe for everyone (in MKS hunting, for Thunder Thrust/Hatchet Man ability, with a 50% chance of killing them with a critical hit), 100 Shield for everyone (Magic Mirror in case you are confronting Fowleye/any Legacy Boss with high spell-casting rate), 100 Virtue for everyone (highers Resilience/Defence and boosts HP by 80), 100 Fource for al least one person (with Armamentalist's Album for casting Fource to everyone), 100 Courage for everyone (boosts HP by 60), 100 Spellcraft/Faith/Enlightenment for spell-casters (boosts Magical Might/Magical Mending, boosts max. MP by 120 and reduces MP cost by 25%), 100 Focus for DDs (boosts their max. Agility and makes them able to use Psyche Up), 100 Guts for everyone (boosts max. HP by 60 and boosts Strengh)
The Proffesional SP allocation can be hard for some people. Allow SP under your own resposability.
Part 3: Item Allocation
Every teammate should have al least one Yggdrasil Leaf, but if you're going to fight a Legacy Boss, you should get 2 or 3 for each member. Also, give everyone some Yggdrasil Jews and Elfin Elixirs/Sage's Elixirs in case your Healer/Spell Caster loses too many MP.
Give your Healer the Minstrel's Manual and the Armamentalist's Album, so he/she will be able to use Gritty Ditty and cast Fource to every teammate. Also, give him/her the Priest's Primer manual in case your team has getted damned. With that manual he/she will be able to cast the Wave of Relief ability, who will take out status like poison, sleep, paralysis and confusion.
Here finishes my Team Building Guide. Hope that I helped you :)