Currently, I like to use a hack that automatically makes all models use attr_accessible with no allowed attributes. Until you override it with attr_accessible in your model, nothing is allowed, so you have to think hard about what should and shouldn't be accessible.
However, it causes real annoyance when not dealing with web input. Applying it to an existing project and fixing everything it breaks is super-annoying.
From my experience, the main (though easily side-stepped) annoyance is when creating or updating records that have belongs_to associations (for example, user_id and repository_id for a commit ;)) programmatically.
For security purposes, you would not set those 2 attributes to be attr_accessible. To create a new record, you then would have to build the record and then set the user_id and repository_id on the record.
Or, you can set user and repository to be accessible (attr_accessible :user, :repository). This is fine because the associated methods expect ActiveRecord objects.