Is it bad to rely on foreign key cascading?

I would say that you follow the principle of least surprise.

Cascading deletes should not cause unexpected loss of data. If a delete requires related records to be deleted, and the user needs to know that those records are going to go away, then cascading deletes should not be used. Instead, the user should be required to explicitly delete the related records, or be provided a notification.

On the other hand, if the table relates to another table that is temporary in nature, or that contains records that will never be needed once the parent entity is gone, then cascading deletes may be OK.

That said, I prefer to state my intentions explicitly by deleting the related records in code, rather than relying on cascading deletes. In fact, I've never actually used a cascading delete to implicitly delete related records. Also, I use soft deletion a lot, as described by cletus.