generally...
I would say from a rule POV they should be kept balanced unles the campaign is intended to be unbalancing... ie.) if the army (in this case made up of primarily wizards) rules the city and takes all non-wizards as slaves then in this case wizards should be possibly given ability to make them slightly more powerful.
in a group balance POV, this can be difficult because all it takes is some bad rolls from one party member, or poor allotment of skill points from a new player to make a team really unbalanced. In the game I'm currently PLAYING in my fighter (at first lvl) became a beast really quick... I rolled 3 natural 18, a 17,14,11 I dumped 18 into str, con, dex, 14 int, 17 cha, and finally 11 wis... so I have a whip and a bastard sword (monkey grip allows me to do this effectively), 2 weap fighting, 2 weap def... chainshirt (total AC 20) I basically trip people, and stab them... over and over and over... its hard to beat with anything that should be thrown at a 1st lvl party.
none the less, I'm off track. A good DM should be able to balance things like this out. by simply giving stronger, good fighting members of the party more phsycal challenges, and better skill based characters good skill related challeges. The Key is the DM NEEDs to know his characters. Dont throw 4 orc barbarians at a group of 4 lvl 1 characters... because ONE of them is strong... on the same hand dont throw 1 civilian at the same group because 3 of them cant fight... the one fighter will make up for the slack and decimate them EVERY time... even if this means when a party is stomping on a enemy group... maybe throw one or two more in to help the enemies... maybe they heard the battle.
For these reasons I said "somewhat important"