I used to love rounding my corners when Stampin' Up carried the Corner Rounder Punches in two sizes. After they retired the larger one I started trimming the larger layer by hand so I could get the right size mat.
Now with the new Envelope Punch Board I can punch perfect mats again. Yay!!! the punch board has a corner rounder in the top which is slightly larger than the 3/16" Corner Rounder punch that Stampin' Up currently carries. If you cut your bottom card stock layer 1/4" larger than the top one you can make a perfect mat.