Adding organic material benefits all soil types. Organic material will break down over time, so add it continually to your garden. Add it before you plant each new round of plants as well as at the end of the season. For new beds, add 3 to 4 inches of well-rotted material and turn it into the top 4 to 6 inches of soil, chopping it up and working it in until the soil texture is as crumbly and even as possible. For more information on composting check out the Urban Garden Center website.
Remember, native plants can grow well in the wild without compost, so it isn't absolutely necessary.
Some rain garden experts also shy away from compost, saying it negates the water-quality improvements accomplished by a rain garden because it add nutrients to the rainwater entering the rain garden.
We converted two rain barrels into composters and bought tumblers to turn them periodically. We compost all of our dead stalks from our rain gardens in these barrels.