Keep your assassin assassins, your pirate pirates and your jester jesters. Once you've got a premium class, don't start goofing around.
I usually like to class change after 8 levels, after I'm close to level 9 so I don't spend too long gaining my first level in the new class.
The first 5-8 levels, I'd just invest in attributes to survive at the beginning of the game. After the first class change though, just save them all and save scum to get 5-6 bonus points if necessary.
As for where you want to settle with your casters, it's going to based on the magic skill they are the most competent at. So like Thaumaturge use alchemistry, sage wizard and nec use sorcery, cleric and I think templars use another skill.
So you're more likely to settle on a class that uses sorcery, meaning that you want to get that skill to a 100 before/during your final class change. Though, you could just as well choose to settle on an alchemistry caster and get it up there, etc, but that's not really a good idea since you've got limited skill points and if your wizards aren't high enough in their magic skill (which is more likely to be sorcery), they'll whif and backfire for a while.
You still have a lot of freedom in deciding what caster you end up with, and by that time in the game, it won't matter that much, it's more about getting the mana points. Though necro get the best spells, so it's not a bad choice to settle on them.
Lethal blow is probably the best thing in the game, I haven't used backstabbing once, it takes too long to set up. But hey, I haven't finished the game yet, so who knows, maybe backstabbing becomes worth it.
I just went with the flow and managed to make two pirates, one of which I switched for an assassin npc. I didn't quite understand the multi-classing paths very well and wasn't really able to do much more though I had the points for it, but it's not like I regret it at all. The game isn't that hard after a certain point combat-wise, it just finds new ways to challenge you.
EDIT: Don't worry too much about optimizing your party. The game's much more fun when you're going for a sub-optimal build. Like, technically, you could probably make a party of jester wolfins of mass destruction and have a piss easy time with the game, but it's not going to be very fun.
You can't really make any sort of game breaking character progression decision if you just follow the rules you already seem to know, half of the fun is in exploring the strange worlds of hyperborea and its masterful dungeons.