My planting system closely resembles Rob's. The only thing I try to do is not place flowers of the same color next to each other (and that includes in front of and/or behind, since all of my beds are multiple rows). Also -- like Tom -- I try to plant selfs next to multi-colored ones.
I'm not OCD about placement -- my joy in my Irises is the individual blossoms -- and not the *landscape* appearance.