Roman letters are designed to sit side by side, not on top of
one another. Uppercase letters form more stable stacks than lowercase letters. Centering the column helps to even out the differences in width. (The letter I is a perennial problem.) Stacks of lowercase letters are especially awkward because the ascenders and descenders make the vertical spacing appear uneven, and the varied width of the characters makes the stacks look precarious.
The simplest way to make a line of text form a vertical line is to change
the orientation of the baseline from horizontal to vertical. This preserves
the natural affinity among letters sitting on a line. There is no fixed rule determining whether type should run from top to bottom or from bottom to top. It is more common, however, especially in the U.S., to run text on the spines of books from top to bottom. (You can also run text up and down simultaneously.)