Protecting OS from pool fill-up

Is it better to set a ZFS reservation on the root filesystems or is it more advisable to set quotas on any/all user/application filesystems that could potentially eat up all available disk space?  With / and /var being mounted under rpool/ROOT-[bename] is setting a reservation on the OS itself even feasible?

This dilemma doesn't seem to be discussed much in best practice documents.  We recently saw an application spin out of control and fill up the pool, to the point of corrupting the SVC repository database.  Thanks for any insight!