The following bit, although on the rather shallow topic of writing computer code, rings true on a great many non-computer related levels.
Models that will be used consistently on all the site's pages are best autoloaded in the
config/autoload.phpfile. Models that are used throughout all the methods of a controller, should be loaded in that controller's constructor. Models that are only needed in certain methods within a controller, should be loaded inside those methods, only.
— From CodeIgniter Optimization.