Psst, don't spoil the surprise
You get the big level 12 bonus only if you reach level 7 and 12 in the same class first, if the classes are different you only get 2 small level 7 bonuses, therefore my recommendation.
Since XP requirements for level advancement are high after reaching level 11 it's time to find a primary class.
The most powerful way is taking the 13th fighter level to get the extra half attack per round and taking only mage levels afterwards. This way you get a mage with all important fighter benefits.
Another way is taking thief levels after fighter level 13 to get a thief with all important fighter benefits who can hide and backstab for big damage while gaining levels fast.
Or you could just ignore the other classes and keep on levelling as fighter to gain stat points faster, maybe invest a few extra XP for gaining mage level 12 (level 6 spells) or thief level 11 (cheap XP requirements).
Keeping the levels even does also work, but this yields the least stat boosts and changing class-specific equipment each time you change your class is annoying.
It's not a bad idea to switch classes on special occasions however, if you're a mage you've to switch to fighter in order to get fighter NPCs (like the fighter trainers you need for spending proficiencies) to teach you or if you're a a fighter temporarily switching to mage and boosting int to 18 temporarily with talimans in order to understand all eight circles of Zerthimon from Dak'kons device is worth the trouble for example.
But don't bother too much, you're powerful enough to finish the game no matter how you develop your character.
Getting +++++ in a weapon proficiency yields an extra attack per round so I recommend putting the proficiency points you'll get (you'll get one at fighter level 12 and the next at fighter 15, thief 16 or mage 18 depending on how you develop your character) into one weapon type instead of spreading them around. I'd take edged weapons since you can use them no matter which class you're currently in.
