Designing a garden with a slight slope

Last year, I created two new flower beds in my front yard. Both are close to the street, which has a slight slope. Because of the slope, I was hesitant to pull up all the grass. I didn't want to create erosion problems by not having plantings there year round. Instead I left swatches of grass between the beds and the street.

Today as I was driving to work, inspiration hit me. I suddenly realized while admiring a neighbor's new plantings that I often see ornamental type grasses planted in areas that have slopes. Presumably, the grasses ward off erosion, and are low maintanence as well. With this in mind, I am questioning if all grasses would do this or do some of the have shallower roots, and thus would not be good options. I actually have an excess of ponytail grass (stripa) that I grew from seed last year. I often see liriope or mondo grass used in our areas.

I think that switching out ponytail grass for the present boring lawn would be more attractive, but I want to ensure that I don't end up losing my topsoil.