I never use a shim to establish toe in. Always hold/position by hand and check as I tighten the pad. But some brakes develop a notch where the pad was clamped and there can be a tendency to have the new attempt slip back into the old position. The brakes that don't use concave/convex washers/spacers are prone to this. So sometimes one has to position the pad in a different enough spot within the arm to get past the indents from the previous position. There are times where I file/grind the pad to establish toe in.

On my own bikes I set up the pads flat against the rim and then either ride the pads in or dirty them up to prevent squealing. Andy.