For tests and small pieces, I used a milling machine to cut the trenches, but for large items, I used the ShopBot CNC router at TechShop. http://techshop.ws
One thing to remember: each bend uses up some of the material (bend allowance). If this is an important issue, run a test. Cut a trench, say 2" from an edge, bend it, then measure. With the .12" thick ABS plastic I used, I lost .05" to the bend.
The trench puts the bend right where you want it, every time.