You can't properly tighten cranks with just a hex wrench. You may think they are tight enough, but they are not. You need to tighten them to about 400 in-lb (33 ft-lb), which requires a torque wrench. Using loctite is unnecessary, and won't maintain the pre-torque. I have Sugino RD2 cranks on three of my bikes (one is a geared double), and have never had any loosening problems with them.