Dear Nathan,
The tricky decision is, as you point out: is it preferable to have mixed line
heights or to have one large baselineskip for the entire document? In my
experience with mixed Arabic and English there is no b/w solution (sadly for
the ideal nice and regular grid layout we desire), and a compromise which
pleases the eye tends to be the way to go.
To control the leading locally for entire paragraphs or pages I use the
setspace package, which allows you to use the environment spacing with any
measure you want.
If I have a mixed paragraph with Arabic scattered here and there, I would
usually keep the Arabic in mboxes (something like \mbox{\RL{}} to make sure
that the paragraph building introduces the necessary extra leading
automatically, thus avoiding manual line breaks.
You may want to make sure that your document has \raggedbottom at the
beginning, to prevent TeX's vertical stretching to creep in and mess with your
leading.
I hope this helps,
Juan
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