My niece and I do the layout and editing for books. Lately, she has started letting her program hyphenate words at the ends of sentences to avoid the rivers of white you see otherwise. This has created lots of problems for hyphenation. Sure, I can look them up in the dictionary, but the dictionary is not always clear about the endings of other forms of the words. For example, is it attend-ed or should it be atten-ded? It just isn't always clear on these other forms of the words. And it doesn't always seem to follow the same rules when you can find examples.
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The comments so far point to the lack of widely accepted rules about hyphenation. If your underlying aim is to avoid rivers of white space, use full justification to yield even margins on both sides. Left justification, also called "ragged right" or "rag right," is widely preferred, but not if you are river-averse. Full justification tends to work poorly for narrow columns, but for most books it's fine.
You are running a commercial enterprise, and perhaps don’t have time for rule phylosophistics. What "http://www.thefreedictionary.com/" gives can be taken for granted, for the time being.