We do not delete on cascade on the model with template because they are referential data that are used on Many2One so the cascading will fail in most cases. Also we have for most of them date range (see issue7417).
But the line models are a little bit different, they should never be used as referential data. So they can be deleted. And we have no date range on them. So if we need to change the lines, the previous one will stay. So the user will have to manually clean the update.
So for me, it is better to delete on cascade those records.