As explained by typographer Ellen Lupton in Thinking With Type, “Roman letters are designed to sit side by side, not on top of one another.” Fonts are painstakingly devised to maintain a flow in the spacial balance between strokes and spaces, and between letters, to optimise readability.
So the simpler, more legible, more modern and professional-looking solution for a narrow vertical layout is to rotate the entire line of text. The mind is still more comfortable reading this than reading stacked text. An anti-clockwise rotation is the norm, meaning the text will read from bottom to top; this is because right handed people prefer to ‘lean’ left.
Does this mean that letters should never be stacked?
No. In fact, sometimes this treatment is an easy way to achieve a retro look. But it should be used judiciously and with an awareness that it can limit readability.
If it is felt desirable to stack letters, they should be all capitals, because lower case letters treated this way take on a precarious look that is visually unappealing. It also becomes necessary to carefully adjust the optical centering of each letter below the last. The less ‘fancy’ fonts work better.