Weather, ATC, weight - it all plays a part. I've been told many times to keep the speed up for as long as possible, especially in the first minutes of rush hour. That means everything goes out a little later than the normal plan, but if the energy is managed properly then it's not an issue when the plane starts getting dirty. Granted, there is a "lower limit" for when you gotta be configured, but that's what managing the plane is all about!