The SIP panels in my home already had the wire channels cut out before the inner wood panel was bonded to the foam. Then they clearly marked on the indoor side where to find the wire channels. Outlet boxes required a little enlarging of the wire channel to accommodate the box. The electricians wired our two story 2400 sf house in two days.

I think you will find that any slotting of the SIPS will damage their resistance to wind forces, no matter whether the slots are vertical or horizontal. There is another relatively easy way to make provisions for wiring, if you plan for those locations before you assemble the walls. You can heat a metal rod and push the rod against the inside surface of the SIP to make a styrofoam conduit through which you can use a pull string to run your wiring. In most structural members you can cut or bore holes in the middle of the individual piece that are relatively small without damaging the overall strength of that member, but any cut, vertical or horizontal, that starts on any edge will significantly diminish the strength of that member and may make the whole section fail.
Bill Hart,
General Contractor and Construction Consultant

Just saw this on green renovation on the DIY channel. It's called hot balling. Basically, before your wall panels are totally set, heat a one inch bearing ball with a torch (hold with plyers) it's got to be HOT. Then drop the ball into the styro filler in the location you want to run wiring. Then let gravity and the heat of the ball takes it's course. The ball will melt it's way through and drop out the bottom. Leaving a one inch hollow just like the ones placed by the manufacturer. Easy as pie!