How important do you think it is to choose which instruments have which parts and what instruments to use in the first place? I always try to set instrumentation in my pieces to fit my fancy, or to fit/create the mood of the piece.

I think instrumentation is a huge part of how the final piece will end up sounding. The tone of an instrument really affects the mood of a piece. I also like using unique instrumentation to try and make interesting new sounds that catch people's attention (something like doubling up electric guitar and oboe on the same part)