Personally, I like incline period & only do incline. The only flat movement I do is flies. I do what gives me the best contraction & pump in my pecs & that happens to be incline. Once in a while I'll do some flat to mix it up a bit. It's about total chest pump as you can not solely train your upper pecs by doing inclines just as you can not hit lower with decline & middle with flat. It's one total muscle & you should do what suits you best. But, like said before, most people do flat first for ego purposes.