Oh, I would never even dream of installing a sub panel without all conductors. I've seen enough sub panels with no neutral to make my head spin. $5.00 worth of wire makes it hardly worth cutting that corner and that's with today's copper prices.
I also agree that it's tacky to have anything but feeders in the nipple (that didn't sound good). I plan on butting the sub panels below the existing panels on both jobs with dual 1.5" nipples. Ouch, that sounds even worse, but you know what I mean from a professional standpoint, right?
