In an attempt to more thoroughly comprehend how brake toe works, this is a post addressing the uncertainty. I get how having the pads angled in toward the front produces a more stable engagement, but how do the brakes maintain this angle? My understanding breaks down when picturing the brake pads wearing over time to match the angle of the rim. Perhaps the brake calipers are designed to allow a certain amount of flex to occur when the pads are engaged, although sometimes the brakes are only slightly engaged which would not be enough force to cause this this flex.

At times, noticing a squeal from a brake, I have taken a crescent wrench to the pad mounting section of the calipers and given it a certain torque to impose a toe, which would solve the problem for a while, though the brake usually seem to start making noise again at some point in the future, which has given rise to the postulations above.

One of my bikes has smooth post canti brake pads. Yup, every few months I need to tweak the toe-in. Although if I'm lazy I'll just blast downhill and ride the brakes. That little extra wear is often enough to reduce squealing. Especially if the squealing was caused by running through puddles or freshly wiping down the rims and pads with cleaner or alcohol to remove oil splatter from a freshly oiled chain. After a few brake uses any minor squealing stops.

FWIW, Kool Stop Eagle 2 pads are easy to set toe-in. There's a wedged shaped heel that sets toe-in automagically -- just mash the pad flat against the rim and tighten the bolt. If nothing shifts during tightening that sets the position, other than the usual three-handed juggling needed to set equal closure gap via cable. Good pads if your frame has enough room.

Unfortunately my bike's front lacks enough clearance between the fork and Eagle 2 pads, so I substituted Jagwire long, curved slimline pads -- also excellent. They did need some toe-in but aren't picky. I use a zip tie around the brake pad heel to keep my hands free to set the other pad and cable adjustments.

Yes, toe in will tend to wear away & need to be reset.

There is often a mechanism to allow this adjustment of the shoes,

& it can be not obvious how it works without fiddling around.