Agreed with Robert, that is a minor optimization but I don't see negative effect SEO wise.
Yes, exact match anchor texts can be powerful, but nowadays search engines rely less on it directly and will leverage text found near the links themselves and elsewhere on the page. Let say having this structure should have a pretty similar impact on giving anchor text weight :
<nav>
More violin sheet music categories
Christmas
Classical
Traditional
</nav>
Besides SEO, another thing to consider is the fact that screen readers will read the text in the alt attribute. Think about how it could help (or not) a user to hear : "graphic link christmas violin sheet music" "graphic link classical violin sheet music", etc.
Hope this helps with an added perspective on alt attributes.
