I'm not 100% about NWN, but there is a style called E6 out there for DnD which pretty much has it that all classes stop progressing at character level 6. So 2 ftr, 4 wiz, or whatever. The idea is that around level 6 is where each class is about where it needs to be to 'Come into its own'. High AB classes like fighter and barbarian get their second attack, wizards and sorcerers get their 3rd level spell or spells, and rogues have enough sneak attack to do well enough in a fight. The idea is that Level 6 is strong enough to defeat what could be considered normal opponents, ya know 1st level fighters and the lot, in quite a few numbers while a number of them still remain dangerous, but they aren't high level enough that they can skate by encounters like liches, mind flayers, and the like. An Iron Golem is still a really tough and scary fight. NWN is a bit different because it would take a lot of scripting to do what E6 does after level 6 for progression, but I liked the idea so much I originally made my server a 1-10 level only. It maintains that sense of danger when a lich isn't just part of the spawn set for an area.
However I found something out. Level 1-10 is much better on paper than it is in NWN. E6 kind of assumes its the age old PnP, once or twice a week game. I couldn't get the XP low enough to slow progression where it didn't feel like a truly mountainous climb level by level in NWN. Plus I can't replicate the E6 ideal perfectly. After a long amount of testing I decided to up to 15th level. This gave my players more room to level up with prestige classes and other builds, which are fun to do even in an RP server. It also let me expose them to still very dangerous opponents while not going overboard that I have to stack them against six vampires and a sock monkey lich just to give them a taste of mortality.
The only other thing I might want to point out is that unless PvP is something your going to be seeing a lot of class balance is good enough as is IMHO, at least in the first 20 levels, like Shadooow said. I wouldn't stress too much thought about it.
Hopefully my experiences might give you insight into whatever you choose to do '> Happy gaming.