I'm enjoying learning from all the responses here, very fascinating thread.
As for me, I lean towards a ragged right border (left align), it's just a bit "easier on my eyes." But it's not a huge deal, just a preference.
And to me one of the hands-down coolest things about e-books is that I can choose to justify text, left align, right align, center align ... whatever I prefer at the moment. Ain't technology grand? :-)