I did the same changes in https://phabricator.kde.org/D15102 at another place and I put the begin and end at the start and end of the routine, like you did. But I remember that I got review asking not to risk to change signal order emission, in case, and put the begin/end just around the m_textDocument assignment.
I'm not completely sure that I remember correctly and I cannot find it in Phabricator anymore...
Wouldn't expect much risk. modelAboutToBeReset is rarely used, grepping Calligra returns only text shape style dialog thing which only repeats the action, and relying on state between begin&end doesn't soud like a good idea.