Actually with progressive weight training, the whole idea is to fatigue a specific muscle group. As a general rule you should be using more weight (percentage wise) on the first exercise for any muscle group (there are exception of course with more advance ways of training). So if a good set of Inclines taxes the chest muscles the next sets of flats will require that the weight used be reduced somewhat. Vice versa if the exercises are changed around. That is the point and that is what you are after, proper fatigue of a muscle group. But not to the point of overtraining, which could delay proper recovery time.
In my view, it really does not matter if Inclines or flat benches are first in a chest workout. With Inclines, the higher degree the bench is the more the anterior (front) delts will come into play anyway, even at the expense of working the upper pecs. With flats, the chances are most of the pec area (even upper) will be worked better if a wider grip and the bar lowered nearer the upper chest/neck area. That is, when a bar is used rather than DB's. With DB's, either when doing Incline or flat, keep the elbows wide out and the DB's inline with the upper chest/neck area.
Another Example: If you want to try the Pre-Exhaust system than do a set of Pec Decks followed immediately by Inclines or flat benches. Which ever appeals to you at the time. If the Pec Deck at your gym is one where the elbows are placed on pads (rather that gripped way out with the hands) you are in luck.You want the point of resistance on the elbows, better direct focus/leverage on the pec's themselves. DB's and even cables (though getting a great pump,it can be misleading...a pump will not insure actual muscle growth) are a second choice when it comes to a more direct focus on the pec's (upper and lower of this single muscle group) with most versions of the Pec Deck.
If you have a need to take it one more click up than start with a set of Pec Decks, than flat benches and end with Incline presses. All in a Tri Set matter. Keeping the rest to zero between exercises. Try two, or three cycles at the mos, of any Tri Set.
Pre-Exhaust is when a extension exercise is done before a major press/pull exercise. Like Pec Deck/Flys before chest. Laterial raises before delts. Straight arm pullovers ( or bentover rear raises) before back. Leg extensions before squats. ETC, etc, etc. Good Luck.
