I usually use incline as a separate exercise - don't do decline much. It does work many of the same muscles, but also some different ones. So if you are supposed to do 4 sets, I would do all 4 at one setting. Keep that the same for a month and then change. This way you never get stuck doing the exact same exercises all the time and you can make better gains. So do flat for a month, then do incline for a month, etc...