I have 6 responsive variations for a web app, from 916px to 320px wide. While running some internal tests, 2 out of 6 people mentioned that they would prefer if the app responded to bigger screens too, instead of stopping at 960px. This group is not exactly representative of future users, who will most likely have 1024 or 1280 px resolutions, but I'm afraid I might be applying what I do to what I think most users will do (I have a big screen, but my browser is never maximised). 

Should I consider a further responsive step to, say, 1260px? Although most people have those mentioned resolutions, is there any data around what percentage uses the browser in 'fullscreen'?