Optimizing for one screen resolution is a bad idea: people do have different screen resolutions, and will want the application to fit
their needs, whether they live on a 1024x768 screen and pull the application window to 860 width because they want room for Skype/AIM/... alongside it or whether they have a 1900x1200 screen and just pull it up to 1400 pixels wide.
It's important to note that most people will not use your application full-screen - if you buy a larger screen, you usually do that because you want to fit more onto it (in the worst case: more Photoshop/Illustrator pixels when you're doing print stuff). Assuming that you can just design for one particular screen size, fullscreen, and stick out your tongue at anyone else, will cost at least some potential customers.