One of the first things you do in a new project is set up the grid lines. These grid lines are then copy/monitored by the structural engineer and assist him in laying out of his structural members. The Architect 'owns' them (in the sense that he/she created them and the other trades copy from the architectural model into theirs) but often times the structural grid is laid out after consulting the structural engineer.
The vast majority of the time, the grid lines are square to each other. In fact, in Revit it's actually difficult to not make it square. Depending on your zoom level, the grid will be square or off a couple of degrees (3-5). If the grid lines aren't square at the beginning of a project, then the problems will compound from there. Walls won't be square that should be and you'll have problems dimensioning.
Moral of the story: Make sure your grid lines are square at the beginning of a project. Unlike the example below. They are close enough to think they are square, but just enough off to make life difficult.
