Never say Never
yes filtering can be inheerantly dangerous but it can be completed without conflict or putting yourself at risk.
the problem with giving a general rule is that every road, queue, situation is different.
so what you need to learn is how to make the decision whilst riding as to wether or not to filter in any given situation.
if you decide to filter then;
keep your speed down less than 10mph passing stationary traffic.
filtering through moving traffic really is another kettle of fish
if it's 6 cars as you say then you want to be sure you can get to the front before the traffic moves off otherwise it's not worth the hassle.
as said if you need to use the opposite carraigeway to filter then ensure you have somwere to go should a vehicle come toward you
if your at the front don't cross the white line but be ready to get away befor the car next to you but don't jump the amber as you could find someone else has done this on the road crossing yours.
as you merge back into the traffic give the driver behind a thankyou wave.
filtering in some circumstances can be essential, on a motorway of fast dual carraigeway as i wouldn't want to be sat at the back of stationary traffic with vehicles approaching your rear at high speed.