John McWade of Before and After magazine discusses fine-tuning text for large sizes. He even specifically mentions posters!
I haven’t yet found the need to tweak letter spacing as described here. For titles and section headings, it might be worth it. Titles can be very large. Most conferences recommend making your title an inch high (72 points), and I’ve often set them larger (90 or even 120 points).
For the body of the poster, it might not be worth it spending the time fine tuning all the spaces between letters unless you can do it automatically.
Thanks for the tip, John, and allowing me to be a lazy blogger this week.
Speaking of text, I’ll take a second to throw in this link to a post on whether to justify text or not.