You’ve just typed in your journaling block, and now the moment of truth: Do I center it? Justify it? Align it to the left? The alignment you choose will affect how someone reads your words. Here are some general guidelines to help you decide.
Align to the left when you have a large journaling block. Why? A left aligned text block (also called “ragged right”) is the easiest of all alignment styles to read, especially when you have lots of text. For this reason, most magazines format their text using a left alignment. When in doubt, align to the left. The journaling on my layout here has many lines, so I used a left alignment to enhance readability.
Centering your text is OK if you have brief text. Centering is great when you are using a quote or other journaling with eight lines or fewer. Pay attention to your line lengths. Centered text blocks look more appealing when the line lengths vary, fully emphasizing the centered alignment. Justify your text with caution! Justifying text, or forcing each line to be the same length, is tricky. Why? Because inevitably some lines will have fewer words, thereby causing them to appear more spaced out than others and allowing for larger gaps between words. This creates an uneven, discordant look. If you have similar-sized words, or you’re comfortable shifting words from line to line to adjust the number of words per line, give it a try.
--From Cathy Zielske via Creating Keepsakes.com
Personally if I have little story, I’ll Left align (or right if it is next to the edge of something), but if my story is very long, I think justify works best. For lots of text it is easiest to read—that’s why newspapers and magazines use it. In PSE, highlight the text and use the shortcut Ctrl-Alt-J