Think about how you want them to play and give them the skills to do it. Its not bad to let Defence lag on some players (after all they might be 5*/6* on attack and physical and still decent defence at 4. But *imo* if you find yourself thinking a player will "cover that flank" or "defend v that player" then at least make sure he's capable.

PS. I *think* the probability of using skills skew from back to front anyway (with a balance in central midfield rank) so that sets a base Raise or lowe Def and Att based on what you want - I like Phys & Men to be at least 'at Q'..