Basically, never ever stack it. Most likely you won't be using a fixed-width typeface, so the letters won't align at all, and readability will go right down the tubes. Set the text just as you would, then rotate it 90°… Which direction you choose to rotate it is more up to you, but generally, it looks better if the baseline is facing the edge of your interface.

Yes, a monospace face, capitals or unicase, such as Panoptica would cause the least problems.

There's a bakery in my neighborhood name of Schick's that has a sign set vertically:
 S
 C
 H
  I
 K
'S

Looks odd, but I guess someone had his heart set on that vertical sign.

—Joel