Well it depends on what I'm training that day...
If the workout is longer FTP type intervals then I will look for a flat stretch of road...If I'm doing V02max stuff then flat or hill doesn't really matter as long as the climb last the length of the interval...
If the goal is an endurance ride with a normalized power in a certain range then I don't worry about it as long as at the end of the day I am in that range...