Dechlorinator is not strictly necessary, however I am not recommending that you shouldn't use it.
Several of our members here don't use dechlorinator, and seem to be successful in doing so.
With regard to chlorine killing the filter bacteria, this is true but the reality isn't as straightforward as that.
Chlorine will damage your bacteria colony, this we know, however it depends on several factors. The most important ones being how much chlorine is present, the age of the filter bacteria, and the the size of the water change. If you changed 100% of the water in a newly cycled tank, you will likely do some significant damage to your bacteria colony. However, if you change 25% in a tank which has been cycled for a year, the effect on the bacteria will probably be minimal. Bacteria colonies strengthen with age and maturity, so the more mature your filter is, the less at risk you are from a disaster.
Also, some strains of our beneficial bacteria are thought to be resistant to chlorine. For example, when you first fill a tank and put a brand new filter on containing no bacteria whatsoever, how does it start a bacteria colony? Where do those first bacteria come from? The answer is they are in your water supply. There have been several studies which have identified nitrifying bacteria living in municipal water supplies. It is thought that they are able to consume the ammonia present in chloramine, leaving chlorine in the water which quickly gasses off at the surface. They therefore must be resistant in some way to the effects of chlorine and chloramine to be able to survive in such conditions.
I wouldn't recommend not using dechlorinator as although the effect on the bacteria can be minimised with some knowledge of what is actually happening, the effects on fish are less known. Many people report healthy colourful fish whilst not dechlorinating, but to play devil's advocate, many people also report happy healthy goldfish kept in tiny unfiltered bowls. I just can't accept that as evidence, and I'm not aware of any scientific studies in this area which we could refer to.
Also, you can never be sure when the water board are going to increase their dosage.
I personally don't think its worth the risk. I use pond dechlorinator which is much cheaper than aquarium dechlorinator. It works out that it costs me around £1 per year. I wouldn't worry about it if I ran out or forgot to add it one time, but for all it costs, I'd rather have the peace of mind it brings.
Hope that helps you.
BTT