Let's fight!
Single space after punctuation when using a computer or word processor because it automatically kerns the characters correctly, whereas those of us who learned to type on a typewriter know that a typewriter does no kerning on its own. Double-spaces say, "I'm typing this on an old...
Single space after periods. Punctuation inside quotes ("art." not "art".). Left justify the text.
All these small things will help your presentation.
Sorry, I'm an editor. It's a habit.
:)
MC