A rule of thumb for readibility is 50% white space. See p. 124 of this article for expansion:
http://www.ca7.uscourts.gov/rules/pa...with_print.pdf

That includes margins and interline spacing.

Although that article is about legal writing, it is one of the best I've seen on producing documents that are read, for any purpose. If you want people to read your documents, you would be wise to keep some substantial margins. If, on the other hand you are just wanting to throw a bunch of words at them so you can say you did it, you don't need the margins. (I'm thinking some of the warning pages that come with products.)