As far as I know, MF (and MP) is not nearly used as often as the non-intermediate dynamic markings at all by any composer.
I browsed through loads of Chopin, Beethoven, Mozart and Schumann, in most cases the Henle Urtext but also some Editio Musica Urtext editions, and learned a small amount of all these pieces, but remember only 1 instance in which MF was used in a work I actually learned: in Schumann's 'Carnaval', right in the opening movement on page 2.