Some musicians insist on sight reading, even if they are soloists in a concert, but even then, I've seen pros suddenly "forget" how to read music in high stress situations. Don't ask me how that works. I've always memorized music quickly and committed it to muscle memory through repetition, but then, too, I've gotten lost, and had to fall back on Jazz training to fake it until the wheels get back on track. One smart ass came up after a recital and said "That was an interesting bit of Chopin modulation in the 2nd movement of the Beethoven."