Theres nothing worse than playing without tactics. I do psychology at uni and i can tell you now that people who know what they are meant to be doing perform better than those who arent sure. obvious enough. Uncertainty leads to stress and quite frankly, when im stressed, i play shit cs.
Tactics are essential. They dont need to be complicated, but everyone needs to know their role and stick to it. If jon terry always came up to attack for chelsea, they would have very little defence...you get me