You have great symmetry, and a naturally balanced build for a pretty well trained ectomorphic frame. How you'll grow and develop with serious resistance training is based more on what is in front of you in the mirror, than on other men.
For example, Chucky is a classic meso/endo, a combination of rounded short muscle bellies and broader, thicker frame with larger joints, in spite of his diminutive stature (which still is taller than me! LOL). In fact, many shorter men in the sport of bodybuilding tend to be more mesomorphic or endomorphic. Me, no. I'm a classic ecto who got fat and is now working my ass off to whip up something half-way decent. Thicker broader guys like linebackers, wrestlers, cage fighters, these guys typically tend toward meso as well.
You, do not. So how to or why to train your upper chest (and yes, Chucky, get a grip, for forty plus years of bodybuilding, even people like Pearl and Mentzer have focused on the pectoralis major's upper and lower quadrants, with the two larger segments of the fan which scallop over at insertion creating a shelf, rather than the cleave distinguishing how the two lower bundles lay flatter toward the sternum, so stop giving the OP such grief... especially when you could be helpful, which I know you intend to be in spite of your spleen...yeesh! When you're shredded, your PM shows distinct separation - admirably - between the two quadrants....man!)
A muscle is best shocked into having to adapt if it's stressed at all levels, and inclines as well as declines do make a difference in overall shape. YOU, OP, are not going to get a big shelf. Don't worry. Not as fast as you fear, and nor to the degree that you find unattractive, at least. You'd have to do some SERIOUS hard work to get that puff on your particular frame. You could, but only in time.
So, the issue is not whether or not to do the particular movement, it's how to do it so that the range of motion is correct, thus giving you real benefit. Inclines and declines with dumbells allow you a deeper stretch, which forces you to engage the pec when exploding up long before you engage stabilizers (delts and tris) Ideally, to keep it nice and strict, a cambered olympic bar is particularly helpful in getting the most out of these exercises. But very few gyms have one. So dumbells are more likely to serve you.
The other great benefit of doing them, properly, is that they make you stronger, which means your flat bench will increase (can't get bigger without getting stronger...naturally) and that is something you should VERY MUCH want.
Do them. A muscle needs a full range of motion as well as targeting beyond the core movements. You'll be glad you did.
And frankly, regardless, you have a terrific build. That's not sycophantish sympathetic flattery. I'm being honest. You're hot as is. No, you're not Chucky's type, but so very few if any of the men here are. It's a cross we bear.