It depends on the recipe. If it calls for cake flour, use cake flour. If it calls for all-purpose use that. A lot of people probably use AP flour in recipes that call for cake flour, and then don't get a good result. AP flour has a higher protein content than cake flour, so you get more gluten when you use it, and the cake will be tougher if you don't adjust the other ingredients.
Basically, just follow the recipe unless you want to make other adjustments after you switch flour types. I've done that before and it's possible, you just have to know what to switch.
Yep, just what costumeczar said.
Would like to add that when using AP flour, bleached flour yields superior results (especially in butter cakes). So that's what I use unless the recipe specifically states "unbleached" flour.
Baking Ingredients Substitution Chart:
http://www.joyofbaking.com/IngredientSubstitution.html
When subbing AP for cake flour, I use bleached AP (because all cake flour is bleached) and corn starch (which keeps the volume the same). Find the results superior TO just cutting back on amount of AP flour.
HTH
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