I would recommend Corner Chompers by WR Memorykeepers. They have several different kinds, which make different corners. If you use the chomper on each layer, you should have perfectly spaced layers on your corners.
Here is a video showing the ones which are available. I have the aqua handle one, which makes two different sizes of rounded corners. If you want an inverted corner, you should use the green handled one. Also, the SU ticket corner punch will make an inverted corner.
Here is a link to a video showing the Corner Chompers in action. I bought mine pretty reasonably on Overstock, but you can find them lots of places.
Product Review: Corner Chompers - YouTube