I clip the inside of the ear leather and use ordinary scissors (not thinners) carefully to trim the length below the bottom of the ear leather to whatever length I think looks in balance with the rest of the dog. I then use thinners on Louis (who isn't clipped at all) and a bit of clipping then thinning on William (whose head and body are clipped) on the top 1/3 of the outside of the ear to shorten and thin it quite a lot.
I think what you do partly depends on what the rest of your Cocker's coat looks like, so that the ears match, and what your personal preference is.