Something that has helped me in using the scatter tool is the triangle or box method. Lets say you want to fill an area with grass. Put one grass patch in each corner of the area you wish to fill. Select all four patches and when you replicate and scatter all of the copies will be placed inside the boundaries of the 3-4 objects you selected. Another thing is to use only the top view when scattering. This allows you to get them all spread out like you want, then drag them up above the landscape and use the drop tool to make them all sit on the ground no matter what kind of contour you have (just make sure they are not grouped, all selected but not grouped).