Same as you bigone, I filter for at least 4 miles each way on my 50 mile round commute. Sometimes more if M4 is being ****.
Have never felt the need to run on main beam at any point. The theory says that if high beam is on then it becomes harder for other vehicles to judge your proximity. Irrespective of that I think it just marks the rider out as inconsiderate.
I've had one incident on that commute when a car changed lanes from the middle lane to the outside lane into me as I was alongside at 50mph. Not much you can do when they don't look.