I think this hardly never necessary. The only places where you should let others pass is under blue flag, accident or tactical decision. If you know that you will lose more time driving in front of the fast guy, then it is a reasonable choise to let him pass. In case of accident, your car maybe damaged in a way that it is hard to predict where the car wants to go, letting people pass might be a good thing there. Under blue flag, of course.
I have raced in LX4 in a mixed grid of LX4s and LX6s and didn't let LX6s pass without fight. Imho, you should always race for position when it's safe and strategically wise. If they are skilled enough they will pass you anyway. In the meantime, enjoy the fight