Good to know I'm not the only one to use matchbox cardboard when setting up brakes.
However I now own the Tacx brake shoe tuner. Effectively does the job of the cardboard, but you clamp it to the rim. Also has a nice lip which helps seat brake pads properly. Works with cantis, vs and road calipers. The tuner is tapered, so as long as you put it on the right way around, it will toe brake pads in.
Really simple tool, makes life a lot easier, can't believe someone like Park doesn't make a similar tool.
Another trick i found that worked well, was to fit the brakes without tyres. Particularly useful when fitting to a frame when you have completely stripped brakes for servicing. Really helps setting them up symmetrically. (So that should appeal to all the emerging OCD behaviour prevalent on here, it did to mine

)