my 2 cents:
I try to avoid callings to qsettings...

in my QMainWindow (or in other dialog)
i read settings related to that dialog at the beggining into variables...
and in the close event i write to qsettings. (or in accept() )

if my main window open a dialog who need read a setting of main window... i'll pass as a parameter.

only I read settings directly when the setting is not specific for any dialog..