Oh, a couple of grinders are indispensable when building a chassis. One with a 1 mm cutting disk, another with a flappy disk (it's amazing how much difference the abrasive compound makes -- get a Zirconia one, it will last at 5-10x longer and cut steel easier). I have a third one mounted in a chopsaw-like holder.
The advantage of a pre-cut is that the first part of your chassis build will be like on fast-forward: 20 hours instead of 60 or so.