Back when I was in college, we had an assignment to create a four season flower garden. The point of this garden was to show the visible appeal of the garden in any given time of year and make us design for that look.
Since that time I have not worked for a client with a keen enough interest to warrant my time and his money to provide such detail. But I always keep it in mind and slip in plants that will fill the year round need for beauty and food.
This type of analysis is vital to the permaculturist as well as any serious food grower. Every farm and garden needs to consider food productivity and total growing power for the entire year. No useful gaps should be forgotten or productivity will go down.