In Wizardry 6:
* Valkyries are really good. They level up faster than Priests while getting the same spells, albeit delayed a couple of levels and with some skill points being forced into Pole and Staff (so try to raise that skill to 51 by practice as soon as possible). The faster leveling means that Valkyries are better casters than Priests in the long run. (Try creating a Priest and then changing her to Valkyrie early; you get the early start on learning Priest spells, but get to enjoy the Valkyrie's faster leveling and better equipment selection, not to mention HP and accuracy.)
* Rangers are really bad. They level up slowly and have horrible equipment (armor in particular) selection; avoid them. If you want a fighter/caster hybrid in your party, choose a different one.
To my understanding, Wizardry 7 addressed those problems: Valkyries don't level up quite as fast (I believe Priests might now level faster than Valkyries) and Rangers got better equipment selection and the ability to critical with bows (I don't know if their leveling speed was adjusted).
Interestingly enough, the Japanese Wizardry spin-off Wizardry Gaiden 3, while having different mechanics (more like Wizardry 1-5), copied these balance issues. The Priest to Valkyrie change doesn't work as well there (in fact, alignment restrictions make it *impossible*), but the relative leveling rates and the Ranger's poor equipment selection are an issue there. Wizardry Gaiden 4 took a different approach to addressing those issues than Wizardry 7 did.
kmonster: - Train your characters in combat, always have your monk and ninja hide to train ninjutsu and fight without weapons to train unarmed and have your bard sing to train music for example.
Note that not all skills can be trained this way; Kirijustu (critcal hit) and spellbook skills (determine what level of spells you can learn), for example, can only be increased via level-up points. Generally, the best strategy is to practice the skills you can and save your level up points for these skills you can't.
Also, note that when you change class:
* Any skill that is at 0 will be lost, unless the new class has access to it.
* Any skill that is above 0 will be kept.
* There are certain skills that are useless after changing class. In particular, weapon skills are useless if you can't equip any weapons that use it, and in Wizardry 6, Music is useless for non-bards. I believe there's at least one weapon skill that's useless for fairies as well.
* Being a high level is not needed to learn high level spells, but it is needed to cast them at a power greater than 1.
* I do not know how the game handles the case of an Alchemist who has the Oratory skill from a previous class.
