To be honest, I don't like the wasted screen real-estate caused by using left AND right margins on large
blocks of text. In Mobi or
ePub. Using both's OK for epigraphs and quotations at the beginnings of chapters—or for songs and poetry, but if it's a letter-type situation (or other special blocks of text) that just need special formatting in order to indicate that "hey, something's different here
"... I'm almost always going to use left margin only and maximize the words/per line by leaving the right-margin "0".
But as always ... to each their own.