I agree, in principle, with Bruce's position that the toe-in will eventually be lost to pad wear. However, if proper toe-in is set in the caliper arms, when the aforementioned condition occurs, the brake pads can be flipped around to regain a natural toe-in, and you wouldn't be removing pad material to get it. Tweaking the caliper arm on these brakes is perfectly safe and no one with competent mechanical skills should fear doing it. As Tom stated, there are tools such as the Park one that are specifically designed for this purpose. While that tool is not my tool of choice for the job, I have nonetheless done the job numerous times. The tool I use is a quality 12" crescent wrench, coupled with a piece of old inner-tube (preferably a thicker, heavier-duty type, rather than one of the ultra-lights - more on this in a sec). Cut a strip of tube big enough to protect the caliper arm from the jaws of the wrench (the rubber tube won't slip like a piece of cloth). Dial the jaws of the wrench down to lightly "clamp" the tube and caliper arm, and adjust, checking and repeating as necessary until the alignment is correct. No muss, no fuss, and no marred caliper arms. For arms that require major adjustment (crash damage maybe), disassemble the caliper, clamp the arm (protected, of course) in a vise, and use the same procedure.
Not trying to be contrary, just offering another viewpoint.
Old inner-tubes are one of the handiest things you can have around the shop (like that Xacto knife and #11 blade). I prefer the big, heavy-duty 26" MTB variety. Great, non-slip protective layer for all types of plier, wrench and vise "interface" on delicate steel and alloy bicycle parts. Trim-to-fit protection for all sorts of clamp-on accessories (that ultra-light might be handy here). Tubes also make great rubber-bands, in any width you care to snip off, that won't dry-up and disintegrate like the office type. Tip-of-the-day.
Wayne Bingham Falls Church VA
BRUCE SCHRADER WRITES: Even if you use the Park BT-3 or similar device and bend the caliper arm without marring the finish, you will still have to bend in again once the toe in has worn off. The natural wear pattern will flatten out and the toe in will go away in time. If you grind the pads at an angle, you won't have to bend the arms...EVER... and the pads can be refaced repeatedly and then replaced when they're worn out completely and the brake calipers will still be perfect.
I will bend the arms only on inexpensive brakes and when I'm trying to do it quickly. If it's a classic brake that I want to preserve, even though it's not a high end brake, I'll still grind the pads at an angle before I bend the arms.
Bruce Schrader San Francisco, CA
=="Not all those that wander are lost." -J.R.R. Tolkien (1892-1973)