I usually focus my team this way.
Human sorcerer- I do this because the humans are not very good warriors, and I want at least one in my party.
Slithzerikai warrior- This is because I base my party around the Slithzerikai, as they are hardy and good on the offensive. Plus, I like to have one pole-man in the group.
Nifilim rogue- I do this mainly to have a balance of all my weapons, as this character serves a wide spectrum of purposes according to what I plan to accomplish. I also get an added bonus with the tool use and nature lore skill, which overall has me in a good mood.
Nefilim Archer- the nefilim archer serves a large variety of purposes, but most of all I just like to keep my archer back on the sidelines, Shooting arrows like the devil and occasionally using a heal spell to nudge the team closer to winning.