Here is some lean UI sketching advice – let the level of detail be a variable in your design process that which you control. Staying conscious of and knowing when to cut a corner or when to spend additional time detailing an interaction, screen or flow is a healthy thing. All sorts of design tools impose certain fidelities on to us the second we pick them up. Take on Axure RP for example and before you know it you’re sucked into aligning stuff at a pixel level whether you like it or not. Load up Adobe Fireworks too quickly and subconsciously you begin writing actual copy, comparing pixels, and choosing RGB color values. The tools which we use, just as Donald Norman said of the artifacts we design, also come with affordances – do stay aware of how much detail they ask of us.