Thinking and planning are fun to do. But you have to be flexible too. You can plan a beautiful line of purple perennials, 16-18 inches high and evenly spaced, along the middle of your bed. A row of nice little marigolds in front of them and a row of pink and white Hollyhocks
behind. When it comes time to bloom one two of the Hollyhocks
have completely disappeared and one is white bloom. One of the lovely purple things has come up a dwarf and another is a nauseating color unlike the rest. And a couple of your nice border of marigolds have gone ape and become waist high shrubs obscuring everything behind them. (But the nauseating one still shows.)
Plan away. Just be ready to go with the flow.