Musicians spend years in solitude learning to perfect their craft, and are rightfully protective of what comes out of the horn.
Musicians are also at the mercy of the composer, arranger, orchestrator, and copyist. If a chart is faulty, it can expose a musician to the stink eye from a colleague or conductor. It will also cause them to not trust the music in front of them and play with less confidence. Thy may not want to play your music in the future, which can affect your career.
A few years ago, Boulder Brass was reading for the first time one of the Mahler songs I've arranged. The second trumpet part switches to piccolo in the middle of the chart. Our friend playing that part that night is an impeccable musician in every way - I don't recall hearing him miss anything. I almost never write for A piccolo so he naturally had his B flat piccolo ready to go. He picked it up and played exactly what I had written on the page. It sounded horrible - totally not his fault. I had written the passage for Piccolo in A and had not indicated that in the part.
I forgot this critical piece of information, and my mistake embarrassed my colleague. We're all friends in the Boulder Brass but my stock took a small hit that day - fortunately, the portfolio is healthy enough to take a few small hits. If it had been the first arrangement I ever put on the stand, it might not have gotten a second reading, and a shadow of doubt would have possibly clouded future arrangements.
Learn to write idiomatically, and proof your work. This includes taking the time to make your parts look clean and professional.
First impressions are lasting impressions.