I would say the the main structure is important. So this would give the main top level groups and possibly sub-groups within them. This lets you see where the different content would be presented. Say it is an artists website, from the home page you might have items like 'gallery of paintings', 'exhibitions', 'work for sale' etc. Within these there might be other groupings, so the 'gallery' has sections for the subject matter (landscape, architectural etc) or may be by medium (oils, pastels, etc).
When you are using a CMS then the structure can be dynamic in presentation to the site visitor as you might allow them to filter for items of interest (landscapes in oils) now the structure is more fluid and not fixed.