Django when is concrete inheritance appropriate

Let me start by stating that I have looked at django-polymorphic for this and still have questions. I have item models and many subtypes for items. Currently, my models look like this.

class Item(models.Model):
    pass

class Television(Item):
    upc = models.CharField(max_length=12, null=True)
    title = models.CharField(max_length=255, null=True)
    brand = models.ForeignKey(Brand)
    screen_size = models.IntegerField()

class Fridge(Item):
    upc = models.CharField(max_length=12, null=True)
    title = models.CharField(max_length=255, null=True)
    brand = models.ForeignKey(Brand)
    stuff_about_fridge = models.CharField(max_length=255, null=True)

I did this at first because then I wouldn’t worry about all of the left joins when querying different item types that would be caused if my models looked like this:

class Item(models.Model):
    upc = models.CharField(max_length=12, null=True)
    title = models.CharField(max_length=255, null=True)
    brand = models.ForeignKey(Brand)

class Television(Item):
    screen_size = models.IntegerField()

class Fridge(Item):
    stuff_about_fridge = models.CharField(max_length=255, null=True)

I am now reaching a point where I realize that I very often query all of the Item models together and have to left join information from the subtypes instead, so I am not really saving myself there. My question is, even if I used something like django-polymorphic, would it make sense to A) put everything that is shared in the parent model and just specific things in the child models or to B) have it like I do where everything is in the child model, but they share a parent model PK just so that they can be queried together?