Are you required to use it when a recipe calls for unsalted butter?
 
 When the recipe is for something rather plain and buttery -- shortbread, for example -- you should use exactly what the recipe calls for to produce the exact flavor that the recipe writer is looking for. French buttercream icing can taste overwhelmingly of salt if you use salted butter. But spice cookies and gingerbread will not suffer, nor will brownies or chocolate cookies (all, in fact, can be made admirably with margarine).