Is it good practice to add indexing to a logical field in a Django model

Question

Suppose we have a model with a boolean field:

class AModel(models.Model):
    flag = models.BoleanField()

Is there a reason to add an index to this field?

I think that this is not the reason, because there will be a small profit in the search (this is only a split of two), but a lot of overhead with the record. But my colleague thinks differently.

Is there any rule of thumb for this?