For most intents and purposes, the decline and incline bench presses are considered "variations" of the flat bench. As with any exercise, switching things up is a good thing. Regularly varying the grips, angles, and parameters of your movements will keep you growing. So after a period of performing flat bench, switch to decline or incline, and vice versa. All three variations work the chest, however ancillary muscles used in the movements is different (Incline bench emphasizes your shoulders while decline uses more tricep).
My two cents is that you might be performing too many different chest exercises (you mention doing flat, incline, and decline, "among others"). My recommendation would be to focus on one or two movements for x amount of time and then switching to a "variation" for a period.
Oh yeah - that guy's "retroactive" comment makes no sense...