Today I finally got my complete 6-axis industrial robot rig (without cables) working like I wanted: one empty, which follows a path, to control mostly all robot axis. I got some wired flippings on the tool bone, so I decided to keep it in FK. Further I noticed that sometimes the robot pose looks a bit strange, but this can be fixed by either placing the root bone newly or by rotating and keying manually the IK bones esp. the arm1 bone. The last method to rotate and key manually the IK bones comes in handy to change the angle of the tool tip.
Rig setup can be downloaded here: http://www.blendswap.com/3D-models/animation/advanced-industrial-robot-rig-v1/
Example of manully rotating and keying the IK bones (here arm1 bone by ca. -50°) to change the tool angle:
before, no manual rotation of arm1 bone
after, with manual rotation of arm1 bone