Ground covers help keep weeds at bay and help keep moisture in the soil. Phlox makes an attractive groundcover as do violets-just don't let the violets take over! Brass buttons and thyme work great also. There are many and you can research to find the one best suited.
you could use mulch under the roses or you can also leave the grass under the roses, as long of the roots are not being blocked or messed with in anyway than they will grow fine.
