A helpful [tutorial on apostrophes](http://www.rightpriceediting.com/rightpriceeditingblog/2010/11/8/the-art-of-apostrophes.html):
> They’re just tiny, seemingly insignificant marks. You can hardly even see them! Well, I hate to be the one to break it to you, but they can be a much bigger deal than you realize.
To me, the edge case is the most interesting: adding the apostrophe-s to a non-plural word that is already s-heavy, such as “more pricks than a seamstress’s thumb.” For display type, I’ll often omit the s. I can’t really defend my choice other than it looks better.
I never thought I’d subscribe to a Christian copy-editing blog, but here we are.