I believe it is fairly essential to have a minimum of 18 in your primary stat and focusing on one build type. IE) As a cleric, I think you should commit yourself to either ranged casting or melee and not a perfect split.
Having said that, I believe things like feats and magical items should help out your weaknesses, rather than further improve your strengths. IE) Cleric has low reflex defense, take Lightning Reflexes. Fighter has melee attacks only, spend a few feats to improve your skills with a ranged weapon and ranged basic attacks so you don't just stand there when faced with ranged foes.
I personally feel it is worth sacrificing the effectiveness of your secondary attribute in order to open up a wide selection of feats. Because if you build someone with a 20(includes +2)-16(includes +2)-11-10-10-8 ability score set, you'll pretty much be denied all those feats that call for 13's which would otherwise help your character.
Instead, going with a 18(includes +2)- 16(includes +2) - 13-13-12-8 opens up a LOT of options. Effectively you'd only have 1 ability score restricting you from feat bonuses throughout the game.
