If you’re a composer, there is a moment when you are laying out ideas for the first movement of an orchestral piece that has serious consequences for all the following movements. You must decide whether you want to use a contrabassoon. Or a bass clarinet. Or a piano. Decisions like this can affect who will eventually play, or even look at, your piece.
I’m told there is nothing more infuriating to a conductor than finding out that the composer has written for contrabassoon for only one movement–and that the part is crucial to the overall feeling of the score. This means that the other movements will feature a fully paid contra player sitting on his hands in the woodwind section. For most of the orchestras that play my work, this is a couple hundred bucks that could be used for something else. I have learned my lesson: I don’t include a supplementary instrument in a piece unless I am going to use it throughout.
As for piano, I have found that most rehearsal halls for community orchestras do not have pianos, and that many concert halls have horrible pianos. Including piano in your score–no matter how Copland-y you are feeling–could preclude having the piece chosen for a performance. You are generally better off writing for marimba or harp.
I resisted using Sibelius notation software at first. I was very happy with my Judy Green score paper and Judy Green pencils. But the software has some features that help me keep music directors happy. Chief among them is the program’s strong preference for consistent orchestrations in a piece. If you try adding instruments midway through, you are asking for a world of trouble. Ten years of Sibelius–and even longer working with the conductors of academic, regional, and community orchestras–has made me a better, more consistent, and more user-friendly composer.