For me some formations prefer either Normal or Attacking.

Attacking formations I attack.
Flexible formations I go normal and then test attacking for a short period if I think it wise then stick with it or change back.
Defensive is if I want to pull out a packed defence or face a much higher Q opponent.
Hard Defending is when I'm faced with much higher Q opponent and I think they are attacking.
Hard Attacking for me is usually when I'm desperate or drunk or desperate for drink.

PS. It could be an old wives tale (btw what's this old wife doing here anyway) but if I start Attacking I feel more prone to the 'First 5 mins onslaught' that sometimes happens so I only do that if I think that there's a chance it'll be me doing the onslaught.

PPS But I've just dropped 8pts out of 9 so don't listen to me. >.<