The popular opinion of designers is to use ragged right edges on text, that is the left side of the paragraph is straight and lined up, while the right side is not, with spacing between letters and words standardised.
However, books, magazines, and newspapers all use justified text, which has straight edges on both sides. They can do this for two reasons - it works better when the columns are narrow with only a few words per row; and they have the power to hyphenate correctly, which allows them to adjust spacing should they need to.
The argument goes that it’s easier to read, especially for those who have difficulty reading, when the spacing is regular. It’s also easier to keep track of what line you’re on when they are distinguished by length.
However, I maintain that text that has 12 to 18 words per line looks perfectly fine, even nicer, when justified, and the spacing is less of an issue at that level.
What say you?
There are some examples on this page.
Plus I use justified text on my blog (you don’t have to actually read it, it’s not very interesting).