Yup. You want level shelves.

Keep the shelves level so they are functional, and keep a reasonable separation between the top shelf and the ceiling, and the bottom shelf and the floor. The closer they are, the more obvious the differences in level will be.

yep, level shelves. And I agree w/ People Space that you want to keep a bit of space so that the unevenness is not that obvious. But also remember that stuff stored there will affect whether it looks bad.

Boxes all the same size across the top will make the unevenness noticeable. Boxes of other sizes, or decorative items, or gaps, will prevent it from being as noticeable.