Hairy,
You will find that the Robbo community will split into two camps here, as it sometimes becomes contentious.
For me with a little over 500 hours on Robbos, and 18 instructors, (continuity training is my thing,) my distillation of the wisdom of these instructors is that careful use of the collective friction in cruise is useful and acceptable to them.
One occasion when it is useful is when the blade rigging is out. Then you will get either a rise (rare) or dropping of the collective when you take your hand off it. A TOUCH of friction will prevent this. Another case is when the carb heat control (not on your Raven II of course) has a friction drag with the collective control. i.e. when you pull carb heat the collective drops. A TOUCH of friction will control this.
You can see the problem. How much do you apply? I was taught to sit on on the ground with the engine off and play with the friction and the collective to learn how much is enough, and how much you can overcome if the engine fails in flight. The tricky thing is that each aircraft is different, but as the owner of one you should be easily able to learn the strength and position of the friction.
Most trainers seem to imply that any use of the cyclic friction in flight is likely to be detrimental to your life expectancy.
FWIW