I dont like to be in the way too much (I know I'm not in the way), but if it would endanger me to be in secondary position, then I will be very agressive and make sure I prevent any other danger to myself.
When I can see that someone behind me is going to impatiently try to overtake me at a pinch point, traffic lights, etc, then I will take a strong position in the middle of the road, and make sure that they cannot overtake. If they hit me, then I would simply say that I was riding in primary to stop drivers from overtaking, and some idiot thought his car was a motorbike.
With filtering and then taking primary, I do it all the time when approaching traffic lights. The fact is: if you are filtering past a lot of cars, then it is likely that they will not pass you for a while. And within that time, the could possibly forget that you filtered past then and therefore not take any silly risks for no reason.