Visually impairment covers a wide variety of conditions that could range from limited impairment to blindness. Someone with a visial impairment may not be able to write, and so would be entitled to someone to scribe for one.
You aren't forced to make use of accommodations, however if - as you say - you sometimes need someone to scribe or read for you then I would recommend that you make sure that your school is aware of this for exams, even if on the day you don't make use of it. The alternative is taht if you decline because you're fine currently, but on exam day feel you need t it may not be immediately available. This can cause additional, unneccessary stress that you can do without.