Neither of my first two steps involves any production tools at all, so I don't think it matters what platform(s) we're aiming for at that stage. I use the same process whether I'm building a web front end, a mobile app, or a native app on a PC. Usually, by the time I'm firing up Photoshop or similar tools, I already have a pretty good idea of exactly what graphics I need and how they are going to fit together.
