I was servicing a road bike this week and this chap needed new pads. He had just finished a long ride in the mountains of Vermont and the descents got the best of the pads by the end of the ride. Aside from a wash, lube, and adjust to the bike I grabbed a full set of Shimano cartridge refill pads from the LBS.
In the photo to the left (rider's left side pad) you can see how caliper and pad flex affected the pad's wear and contact against the rim's braking surface. The pad would slightly pull away from the braking surface at the leading edge due to flex at the brake arm and some from deformation of the pad during hard braking. Some mechanics would say the pad was ducking out or toe-ing outward. Toe-ing in can reduce the chances of brake squeal and if the pad trys to toe outward, it self-corrects due to the intial toe-in; providing you with more pad contact against the braking surface of the rim.
I'm using a penny, a dime will work as well, to toe-in the rear pads on the bike. I repeated the same procedure at the front caliper as well. Yes I lowered the pad in relation to the brake tract after snapping the photo above. While squeezing the brake lever firmly (not Kung-Fu grip style) to hold the penny in place, use a 4mm hex wrench to loosen the fixing bolt on the pad. I typically have 1/4th of the pad shimmed by the coin. Once loose, squeeze the brake lever a bit more firmly and tighten up the 4mm fixing bolt. Watch that the brake pad doesn't twist out of alignment against the brake surface while securing the pad bolt. Repeat for all pads.
The new pad is toed-in and ready to go.