tko,
The standard procedure for the past 100 years has been to use tank water because it's assumed that since tank water has bacteria you will keep the bacteria colony intact as the tank water pours over the filter media. It's also assumed that tap water chlorination kills the colony. The thing is though that bacteria multiply with astonishing speed. Have you ever seen videos of reproducing bacteria? They split geometrically so that one becomes two which then becomes 4, then 8 and so on. There are billions or even trillions of bacteria and they populate every cubic millimetre of the tank, glass, water and submerged surfaces including substrate, not to mention the inside and outside of the filter media. So even if you wash away or kill a few billion, in no time flat the population is back up to original levels.
When you are starting up a tank from scratch this is a different story. You are starting with a population of virtually zero so the geometric multiplication sequence takes a few weeks to go from virtually zero to trillions of trillions. Can you see the difference? This is why it's a good idea when starting a tank to put mulm and organic matter in the new substrate and to "seed" the new filter with water or mulm from another filter to get the population level up a bit faster. Once it's up and running though there is nothing to worry about because the basic population is already very high.
When you install your new filter the filter media will be devoid of bacteria initially, but the tank water is populated, so as it enters the new filter the bacteria start adhering to the surface of the media. It takes a while for this colony to build up on the surfaces inside the filter.
The reason we use chlorine removers is so that we don't kill our fish. When you have a tragedy with a tank it's very easy to correlate things that you did with the tragedy. Biochemistry has so many variables that there are a lot of false conclusions drawn on the basis of apparently correlated events. The classic case of mis-correlation is the one of nitrate and algae, yet we here know (hopefully) that there is no correlation. Optical illusions abound because of the number of variables.
I wash everything with tap water using my garden hose because everything I have is big and there is no way I'll be faffing about with buckets of tank water to gently wash this or to carefully rinse that. I don't have any problems. There are more important things to worry about, like CO2 injection rates and dosing schedules and lighting, and scaping. If you turn your hobby into a venture of rocket science you are less likely to keep it up because it becomes tedious. Concentrate your energy on the tough issues and cut corners on the easy stuff.
Hope this makes sense mate.
Cheers,