Well I've played both games many a time over the years, and my party balance usually goes something like this:
1 Paladin, for the special Paladin only equipment and dialogue options
1 Fighter or Barbarian (for IWD2)
1 Cleric (or Cleric/Ranger, explained below)
1 Wizard or Sorceror (but usually Wizard)
1 Rogue
1 Bard or Druid
I don't like to skimp on the martial/rogue/arcane/divine mix too much, because there are plenty of times where you'll really want at least one character for each situation. I try to mix up alignments too, because there is a lot of equipment that requires certain alignments, including evil. I also mix up race and gender, as much as I can. 2E wasn't very forgiving about using unusual races in certain classes. If it was even possible, there'd usually be some stupid stat-restriction.
So I'll use two lawfuls, two chaotics, two goods, and two evils, mixing and matching in order to fit them all into six characters. Three males and three females as well. Oh, and I'll usually mix and match ranged and melee to even amounts too. What can I say, Arundel's speech about balance really got to me.

Oh, and to any who might not know: in the original game, if you have a multiclass Cleric/Ranger, they get Cleric
and Druid spells when they level up in Cleric. All divine spells rolled up into one! Of course the downside is that they level up much slower than single class Clerics, denying you access to the most powerful spells. There's also the fact that only half-elves get access to that multiclass (maybe elves too, not sure), which always bugged me.
IWD2 was almost too long to really get through for me. I always start getting tired of the characters around chapter 5, but I always enjoy chapter 6 immensely, so I try to muscle through it.