The decline is a great lift, you can really overload the upper body with an enormous amount of weight. Also, from my own experience and from most people I know, the decline carries over tremendously to the flat and incline.
I would never push flat/incline/decline in the same training block, you can only get good at so many things.....all are great lifts though, so long as they're trained correctly (progressively rather than idiotically) and kept into perspective (ie, they're an upper body press, not the be all and end all of weight room prowess ,lol)