Found a quote in Gartenberg's 'Mahler', pgs 299-300:
'Blaukopf points out another significant development beginning with the Fifth Symphony; viz., Mahler worked with orchestral conception from the outset of the work, doing away with the piano, on which he had previously worked out initial conceptions (a habit which most composers maintain to this day). Mahler advised even Marshalk not to compose "from the piano". This remark is fascinating because another composer - in another field and practically another world, although he was a contemporary of Mahler - had expressed the same thought. Johann Strauss [iI], who shared with Brahms the habit of composing at a stand-up desk, shared with Mahler a disinclination to compose at the piano. ("I never compose at the piano. The piano has the habit of making you do what it wants you to do.") Mahler's emancipation from the piano changed twentieth-century orchestration habits, because musical imagery no longer had to be transposed from the keyboard into the orchestra.'
I don't think it's necessary at all to become a master of any certain instrument. Especially after Mahler, if Garternberg's mancrushy biography shows anything, knowing what an orchestra can do, the limits and possibilities all the various instruments, is enough to compose great things - along with theory, etc. An instrument might come in handy if at first you don't have a starting sound in your head, but with today's tech - sound libraries - it almost seems unnecessary unless you plan on being some kind of concert-player or member of an orchestra.