All players need some aggression or they won't do anything, it's like the switch that makes players do what they are thinking of, e.g. A defender looking to tackle someone will be using marking & positioning to be in the right place, anticipation to predict what the attacking player will do, decisions to decide what to do, composure to keep his cool & do the right thing, then his aggression will make him do it. His ratings for strength & tackling will determine if the tackle is successful while his ratings for dirtiness & sportsmanship will have an effect on it being a bad tackle.
I imagine there's a bit more going on than that, but that's the basics of it.
As far as best tactic goes, the best tactic is the one that works for your group of players against the opposition. The main question about aggression would be; do you want a team that tries to win the ball & put it in the net or do you want a team with no fight?