Jobst Brandt figured that brake pads should eventually become toed-in with wear. Consider that if the brake pad starts out perfectly flat to the rim, and the calipers twist a little under braking, it's the rear (leading) edge of the pad that will wear more. This is when squeal should theoretically be the worst, but if you keep going with it, the pad should wear so that its surface is nicely parallel to the rim under hard braking, and is slightly toed-in under light braking.

That said, I use a very thin shim (like a rubber band) when setting up fresh brake pads, 'cause I'm impatient and don't want to bug people around me with noisy brakes any more than I have to.