Over the tops of hills (when everyone has that momentary breather at the top) or when turning into crosswinds are the best spots to attack. This is textbook stuff. Hills and crosswinds are what smashes fields to bits, not head or tailwinds. Make your attack count too. Don't put in one of those wussie 50m jumps down the road. Look at pros like Armstrong and Rebellin when they attack. They put the gap in first time every time.
Rarely would circumstances dictate that its a good idea to attack into a headwind.