Here is a great article on alignment.

Type Alignment

• Default writing techniques will use left alignment to create easy to read text for the reader. Casual letters, unpublished manuscripts, and basic paragraph styles tend to fall into this category.
• Center alignment is used to draw attention and is used a majority of the time for Headlines or Titles. Newspaper headers, book titles, and report titles are excellent examples of center alignment.
• Right Alignment is a clean crisp professional look and is used quite a bit for corporate business letters, return address labels, business cards and a variety of other applications where a formal style of alignment is needed.
• Justified alignment is usually reserved for newspaper print and body text for textbooks, and is more difficult to work with. This type of alignment creates perfect alignment on both the left and right margins without regard for the actual characters. This can lead to the creation of “rivers” of white space throughout the text body. If this happens, reduce the tracking gradually to correct the illusion.