Keep It Consistent, Except ...
Leading, like so much in typography, is about rhythm — and as with a piece of music, you want your rhythm to be steady and unfaltering. The best way to achieve this is to set the leading values within Paragraph Styles. Should you need to change the leading values, you can edit the style definition rather than work on the text locally.
Figure 4.17 Using Auto Leading for inline graphics ensures that the height of the line grows to accommodate the size of the graphic.
When it comes to fixing widows and orphans, don't mess with the leading. You have other tricks up your sleeve — rewriting, tracking, discretionary hyphens, forced line breaks — to fix such problems. Tempting though it may be to tighten the leading a little bit here and there, your document will suffer if you do. Always keep your body text leading consistent, otherwise the rhythm of your type will wander like the beat of a distracted drummer.
Also, don't be tempted to go for the quick 'n' dirty solution of using vertical alignment, which increases the leading in a short column to make it bottom out (i.e., end on the same baseline as other columns). While columns of uniform depth are usually preferable in continuous prose, InDesign CS5 can now achieve this with the Balance Columns feature, which will adjust the height of all columns, rather than just extend the shortest one.
Figure 4.18 Good leading gone bad: The columns are balanced, but at the expense of inconsistent leading across the two columns.
Figure 4.19 When using a baseline grid, the grid increment will trump the leading value, as in the right column. Any increase in the leading value causes the lines of the paragraph to snap to the next available grid increment.
Ultimately, it is our eyes we should trust and not the math. There may be times when you need to relax consistency in favor of optical leading and tweak the leading of individual lines to make the leading appear more consistent. Such a situation may arise in display type, for example, if one line doesn't have descenders.
Figure 4.20 Using optical leading: In the example on the right the leading for the fourth line has reduced to compensate for there being no descenders on the line above.
Figure 4.21 The Skip by Leading option pushes the line after the graphic down to the next leading increment. However, if cross alignment of baselines is what you're after, you're better off aligning your text to a baseline grid (see Chapter 15).