Is it rude to take the lane, then filter when encountering congestion?
In general, yes it is rude. But there can be times when it's acceptable.

I'm sure we all understand that you are trying to maximize your own safety. It is a tricky balance to achieve. Especially since how you ride depends on the local driver and cyclist cultures, and local laws.

In many places we cyclists struggle for respect and acceptance by the motoring majority. There are multiple overlapping factors / factions involved:


The might is right faction. For whatever reason: permanent inferiority complex, transient anger, whatever. Logic will never prevail here, but don't give them a reason to target you. 
The cyclists don't obey the rules faction. Reinforced every time a driver sees a cyclist run a red light, ride on the footpath, or in many cases / places - split lanes, take up a whole lane, ride without a helmet, ride with no hands (on the handlebars), pop a mono, have fun, you name it. To them, if a cyclist does anything they as a driver can't do then it should be banned: the rules are written and unwritten.
The cyclists are unpredictable faction. These people may appreciate that every cyclist is freeing up road space, but cyclists vary in experience and predictability. And in peak inner-city traffic it can be hard to know where we are: "Did I pass him yet? I can't see him; did he turn off? I don't want to hit him ...".


In summary: obey the local rules.

For example, I ride in 4 different conditions, when I'm not in a bike lane or on a bike path:


Inner city, generally with heavy and congested (0-40kph) traffic. The drivers generally see lots of bikes, and have learned to accommodate. Because their max speed and mine match, I take a full lane when it's available. At lights, there are "bike boxes" painted on the road in front of the stopped traffic. Bikes are expected to start from there, so that they are visible to the traffic behind. So bikes often filter through the lanes when the traffic is stopped. This example supports taking a lane, but it is a limited case.
Inner city, traffic dominated by recreational and fitness cyclists (Kew Boulevard, for those who know it). The speed limit is 50kph, so on the downhills we usually take the whole lane. Elsewhere, we mostly use the bike lane, unless there is too much bike traffic to fit.
Outer city (the 'burbs). While the traffic here has greatly improved in the last decade, taking a lane is putting your life in their hands. Some drivers are very bike aware, completely changing lanes to overtake. When other drivers see that behavior, they often copy it. But some drivers can look you in the eye and just not see you. Taking a lane is dangerous because to these drivers you simply don't exist. It only takes one to take you out.
Outer city on regular cycling routes, with no bike lanes. If alone, I ride about a meter out from the curb, to avoid all the broken pavement and any rubbish. The local law says vehicles must drive "as near as practicable" to the side of the road. I only take a lane when it would only be for a short distance, and the motorists are aware of me, and safety depends on it. If in a bunch, we would take a whole lane unless it was unsafe.


The third example sounds more like your case. The other examples show how the local conditions and expectations can change things.