No matter what you've read elsewhere on the internet. It is NOT necessary, and will only serve to make work for yourself and is BAD for your system.
Many items in a boiler system are FERROUS (IRON) materials. Iron RUSTS, but ONLY when there is OXYGEN and WATER present. For RUST and CORROSION to occur there must be three things:
> Metal that can corrode or rust
Take away any ONE of the three and no corrosion will occur. The old smelly water in your boiler system has VERY LITTLE oxygen in it due to it's being heated over and over. The automatic air vents on your system release this oxygen (and other gases) when it comes out of solution from the water. In other words, the water in your system is INERT after a short time.
Fresh water is LOADED with oxygen and other gases... AND minerals like calcium and magnesium.
Every time you flush a system you are re-introducing the oxygen and further corrosion occurs.
The MINERALS are also bad. When the water is heated, they also leave the water and form deposits on surfaces that are intended to transfer heat. The minerals act as an insulating layer and reduce the heat transfer.