Learned this one the hard way as well. The design looks great in august or if you leave the foliage stay from the previous year. Otherwise there are the giant gaps until the soil heats up.
This brings up the challenge of dealing with maintenance as part of your design. When I am designing a garden for myself I use lots of cool season grasses since I like the way they look, but I leave the foliage and prune it back when growth starts the next summer. While a client or maintenance company wants to go through and level the planting in the fall, losing all the attractive winter and spring interest from the previous years foliage.