No, sorry, not in the Virginia area, I'm smack in the midst of "Tornado Alley"; Wichita, KS. Most of my residential/commercial construction is in KS, MO & OK, but I've also done some in Greece, Sweden, Belize, Honduras, St. Thomas, V.I. & "points unknown" between these; remodeling 2 charter yachts while underway in the Gulf of Mexico.
From your replies, I assume this is for a new home, not something like a detatched garage, outbuilding or shed?
With that in mind, I'd personally never consider not sheathing the structure in some manner, even if it were with a non-structural sheathing. The 2X6's are great; 24" o.c. will be fine, and even better with a structural sheathing like a 7/16" OSB. If you were not sheathing the structure, I'd absolutely go to 16" o.c. with my stud framing.
The relevant part of James Hardie's installation instructions read:
"Hardiplank lap siding can be installed over braced wood or steel studs spaced a maximum of 24" o.c. or directly to minimum 7/16" thick OSB sheathing. Hardiplank lap siding can also be installed over foam insulation up to 1" thick.† Irregularities in framing, sheathing, and/or foam insulation can mirror through the finished application".
That last sentence pretty much says it all, to me. I tend to "overbuild" personally, and have never regretted doing so; it sounds like whomever has told you it should be over sheathing thinks in a similar way. As to the HardiePlank itself rolling, I don't think that would be a problem, but the "mirror" effect the installation instructions speak of could be.
Further, nailed to 24" o.c. framing with no sheathing, I see a vulnerability to damage from impact that would otherwise not be there. Even someone leaning against the structure under these circumstances could mean popping nailheads in drywall on the interior.
Bruce, if it were me, I'd either sheathe the structure or move up to 16" o.c. stud framing. Again, I'm assuming we are talking about a residential dwelling, not a shed.
Post back as to whether this is a home or other structure, and any further thoughts/questions.