An anxious moment in model design is the first time seeing the work on the desk, in actual three dimensions. Are the proportions right? Do the panels fit well and lend themselves to a precise build? Does the overall presentation "feel" right?
As the build progresses, changes to tabs and parts are noted and implemented back into the source model or the parts sheets.
It's a rough build done quickly on plain paper, so a sort of worst-case-scenario in terms of construction. If the design holds together here, it's good to go.