All-purpose isn't necessarily actually all-purposeful, but it is a good standby in your kitchen as most recipes can be adapted to use it. The key is really to buy it unbleached. Mushet says that bleached flour can have off flavors and doesn't absorb water as well as regular flour.
When it comes to low-protein vs. high protein flour, you need to look at what you're making. Cake flour lacks the proteins that make strong gluten, making it perfect for cakes and other pastries where you want a very tender crumb. At the other end of the spectrum is bread flour, which has a high protein content perfect for forming the gluten needed to make chewy, elastic breads.
When it comes to substituting one flour for another in a recipe, Mushet says to pay attention to the weight of the flour. Baking is really all about ratios, so when substituting flours, you want to make sure that the weight of the ingredients (and thus the ratio) stays the same. For instance, all-purpose weighs more than cake flour, so a cup of each flour would have different weights.
Mushet also had some great tips on rolling out pastries and keeping pie crusts from shrinking in the oven. To get these tips and the rest of Evan Kleinman's interview with Cindy Mushet, head on over to the Good Food website to listen to the entire episode!
• Good Food with Evan Kleinman from KCRW