I am a big fan of ornamental grasses. Let me just say that first off. I like them so much I used them in my company logo and I often design gardens around grasses, either solitary ones or large meadow plantings. Tough and resilient, ornamental grasses (and bamboos) create needed "softness" and texture when planted adjacent to the often hard edges of homes and other structures, look amazing around pools and water features, go through dramatic changes in color, form, and flowering (their seedheads) depending on the weather and season, and they are easy to care for. I simply like to look at them: backlit by the morning or evening sun, undulating in the wind.
One of the great benefits of planting ornamental grasses in your garden, from a design standpoint, is their ability to create rhythm and repetition in the landscape. We've all probably seen yards and gardens that display an impressive variety of plants but have no real sense of design or continuity. What's missing, in many cases, is a plant or two to "seam" all the various elements together. This is where grasses shine. By casually dropping in a carefully chosen grass, you can create a landscape that flows and is more easily digested by the casual or even focused attention. Of course, you can create this repetition using other plants than grasses, but ornamental grasses (and plants with long, slender leaves like iris, daylilies, flax, etc.) just seen to be born for this role in the gardens they are often found.
With that said, it's often nice to feature an ornamental grass as a specimen, too, or a bamboo. There's no fast set of rules. However, I find it dangerous to have, say, only one ornamental grass in your garden or bed. It just looks odd all alone, much like it is, I think, to plant only one palm in your yard. I think it's because these plants never appear this way in nature and, instinctively, we know this. Ornamental grasses like Miscanthus sinensis (Maiden Grass or Japanese Silver Grass) and its many beautiful cultivars are at their showiest when planted in random groups and repeated every so often throughout the garden or landscape for dramatic effect. I particularly like 'Morning Light' Miscanthus around pools and the dwarf 'Adagio' in smaller spaces. Both need full sun to reach their maximum glory.
Native Texas bunch grasses, too, like Little Bluestem (Schizachyrium scoparium) and Sideoats Grama (Bouteloua curtipendula) work much the same way and are pretty side-by-side. Big Bluestem (Andropogon gerardii), also called Prairie Tallgrass, is one of the original native grasses to Texas and the Great Plains and Prairie regions of central North America. It can grow to a height of almost 10 feet. All three of these grasses are native and are best suited for open, transition areas of the landscape. If your yard backs up to a greenbelt or nature preserve, think about planting these grasses near the borders to invite songbirds and other wildlife into your yard.
One of the ornamental grasses I discovered in the past decade and like a great deal is Switchgrass (Panicum virgatum). Many nurseries and garden centers in Austin sell varieties such as 'Heavy Metal', 'Shenandoah', 'Northwind', and 'Cloud Nine'. All are handsome, somewhat vertical growers by nature (instead of arching or pendulous) and come in a variety of leaf colors and eventual heights. 'Cloud Nine', which is becoming increasingly difficult to find, can reach as high as 8 feet. Both 'Cloud Nine' and 'Heavy Metal' have a bluish leaf color which makes them a pleasant addition to the generally green color of other ornamental grasses of similar size.
There are many great books in print about ornamental grasses and bamboos and their use in the garden. I gravitate to the books by U.K. author, Noel Kingsbury, particularly Seedheads, Grasses and Bamboos and Designing with Plants. Kingsbury tells you how and why grasses work from a design perspective, and I like that. There are also two great encyclopedias of ornamental grasses: The Encyclopedia of Grasses for Livable Landscapes by Rick Darke and The Encyclopedia of Ornamental Grasses: How to Grow and Use Over 250 Beautiful and Versatile Plants by John Greenlee. There is also a recent, important book by Stefan Leppert called Ornamental Grasses: Wolfgang Oehme and the New American Garden that is worth the stiff cover price ($45). Oehme and his design partner, James Van Sweden, are pioneers in the New American Garden movement and their previous book, Bold Romantic Gardens, is a wonderful reference book for the garden library. Sweden has many more. At the end of Ornamental Grasses, Leppert has composed a list of Wolfi-plants (ha, ha) that possess Oehme's sought-after characteristics for plant selection: stability, robustness, flowering duration, and competitiveness. It's an education.