While leading has existed since the days of metal type, Auto Leading is a relatively new concept, emerging with desktop publishing in the mid-1980s. Auto Leading allows InDesign to automatically assign a leading value to the text you set, based on the type's point size.
The advantage of Auto Leading is convenience. You can change your text size as many times as you like and your type will always be readable. As your font size increases or decreases, so does your leading. Although this can be useful when experimenting with type sizes, Auto Leading will probably cause more problems than it solves.
Don't by seduced by Auto Leading. Here's why:
Figure 4.9. Inconsistent leading caused by a single word in the paragraph set larger (example A); the leading converted to an absolute number (example B).
Figure 4.10. Changing the Auto Leading Amount.
Trust your eye, not your software, to determine how much leading you need. Auto Leading is useful when experimenting with type sizes, but when you decide upon the size you need be sure to convert your leading values to an absolute number, even if the Auto Leading value is the same as that number.
Figures 4.11A and 4.11B. Auto Leading applied to a headline (A). While Auto Leading works adequately for the body text, in the headline it is disproportionately large (B).
Auto Leading and Inline Graphics
Auto Leading is useful when using inline graphicspicture frames that have been pasted into a text frame, usually on their own line, and move with the flow of text. Inline graphics are a hybrid between text and graphics: You control the spacing of inline graphics using leading and Auto Leading ensures there's always enough space for the graphic on the line. An Auto Leading value of 100% works well for most inline graphics.
Keep It Consistent, Except
Part I: Character Formats
Going with the Flow
Getting the Lead Out
Kern, Baby, Kern
Sweating the Small Stuff: Special Characters, White Space, and Glyphs
OpenType: The New Frontier in Font Technology
Part II: Paragraph Formats
Aligning Your Type
Paragraph Indents and Spacing
First Impressions: Creating Great Opening Paragraphs
Dont Fear the Hyphen
Mastering Tabs and Tables
Part III: Styles
Stylin with Paragraph and Character Styles
Part IV: Page Layout
Setting Up Your Document
Everything in Its Right Place: Using Grids
Text Wraps: The Good, the Bad, and the Ugly