If you have beds that are 4 feet or more deep, then you have the potential to add a vertical dimension to your garden, which can be achieved by planting low plants at the front to offset the higher plants at the back. Some of the woodland flowers fall back into dormancy by the middle of summer. If you plant those, have something else ready to take place later on in the growing season. The woodland plants are often spring flowering while those that need plenty of sunlight are summer flowering.
If you are doing some version of prairie style planting in your garden, it does not really matter what you have at the front of the border. In a typical suburban garden, the front of the border is actually important in setting the tone for the whole bed. People notice the details in this part of the garden. In general, I prefer to make the patches in the front of the border wider than the patches behind them. The flowers in short plants are often small and larger patches are needed to make them noticeable. If you are looking for really low plants that form carpets, then consider Houstonia or Calamintha arkansana or Antennaria. Hanging baskets are a good way to have low annual plants when you are short of space.