Any guidelines on allowing spells from non-official sources?

[Kae'Yoss]

In my game, they can freely use any spells from other sources - but I must approve of it first (problems with spells are quite rare). Spells from the Core Rules and other general/GH supplements from Wizards, as well as FR - related material are more or less game (it's a FR CotSQ campaign). But then, I'm very generous in these things. But this will change in the next Campaign, which will be a Midnight campaign.