I've never heard that story about Bach, although Peter Schickele
tells a similar story about his comic creation "P.D.Q. Bach".  However,
it _is_ a true story about Muzio Clementi (1752-1832).  After his death
his manuscripts went to his nephew, P. Clementi Smith, who (to his
horror!) found that his cook regarded them as waste paper and used them
not just as wrappers, but as cooking parchment under pies!

Eric Stott
Albany, New York