I've designed a handful of web apps now and I seem to be just as confused about document widths as I was before I ever started. I've made the mistake of mocking it up at 1920 wide and jamming it full of content.
For over a decade I designed countless centered websites at a fixed width that was very easy to determine. 720 then 960, etc. But now I find myself designing A LOT of responsive full width web apps. Which forces me to design the "desktop" version at some obscure min width. Fortunately I've had good data provided by some clients which allowed me to design for the largest segment of their users. But what is a safe/good min width to use in HIFI photoshop mockups? It seems that a lot of dribbble shots are 1280px which is what I've been using as of late.