On my finish crew we have found that having the electricians tack their romex onto the sole plate where they want it gives us the opportunity to adjust horizontally for centering between casings, hearths, etc. We use a 2 3/8"x 3 7/8" cut out jig indexed off the top of the base.
After cutting the base to length, and while the base is still on the miter stand, we measure horizontally for location, place the jig on the base, scribe the four sides of the box, drill holes at two corners, caddy corner to each other and have at it with a jigsaw.
It seems to me that having to secure a jig for routing would take a good bit of time, but having never seen it done I'm only guessing. The system as I've described works great for us, but YMMV.