Disc rotor area swept by brake pad: does alignment matter?

One of the disc braked bikes I have has the older non-series Shimano road bike calipers that take a brake pad rather longer than the more recent callipers of 105 and Ultegra. These pads develop the syndrome you describe - a lip of unworn pad forms on one corner, over time, where the pad projects slightly outside the track of the disk as it rotates.

As long as you don't change wheels to a set with discs that are slightly larger in circumference (even by just a teeny bit) the pad-lip doesn't usually seem to affect braking. As another mentions, such a lip is an easy way to judge pad wear.

I did, though, notice an additional squeal and shudder after braking for a long time down very long steep hills, typically in winter when one is more wary of hooning down the long and bendy steep back roads, as they are muddy, wet and otherwise inclined to slide a fellow into the dry stone wall if he hoons and leans mightily.

Presumably the discs heat up with a lot more or continuous braking, so expanding their circumference by a teeny amount, causing the unworn section of pad lip to be hit by the disc outside edge .... ?

in theory there could be uneven wear with the rotor arms brushing the inner radius of the pad, however i've had tihs on a few bikes when i've gone to 203mm rotors and it's never really been an issue. a lip will eventually form on the rotor due to wear, but this would happen anyway.

you could stick 1mm flat washers between the mount and caliper if it's bothering you, however best now to wait until you change discs and pads again or you'll have the uneven wear issue.

I don't do this, I suspect it makes very little difference, although you might increase your pad contact area by 5-8% which could improve braking (and pad life).

On the plus side, the misalignment provides a visual indication of how much rotor was worn away.