said by Jack_in_VA:Posts and edits must have crossed. Since I don't usually feed-through, I had to find the text.
They don't have to be pigtailed on non MWBC circuits but:
the neutrals must be pigtailed on MWBC installation.
300.13 Mechanical and Electrical Continuity Conductors
(B) Device Removal In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.