This fixes the simple case when all pages are to be printed from the
same paper tray. The use of this setting is still confusing when there
is application-set tray for a page (e.g., for a page style in Writer,
through Format->Page), because the change in Preferences is applied (and
will be used for pages without application-set tray), but Preferences
shows the application-set value on second try again...

IOW, it still s---s, but it s---s a bit less .-)