Carm - I have a Kreg jig and have recently used it to make frames to
support 8 mesh hardware cloth for mite detection/collection at the bottom
of my hives.  These are horizontal frames with the same outer dimensions as
a hive box.  The tool works well for this.  I would not see any benefit or
reason to use the Kreg jig if you are going to make boxes that will be butt
jointed - just screw through the cross grain of one side into the side
grain of the other.

Jim King