Guys,
They (CIC) do not want to force people to pay tax or show tax docs when they never paid the tax, they are actually trying to verify your employment through these documents.
So lets say you have said on your application that you have 10 years of experience in such and such company and you worked in the US, they want company letter as well as the Tax documents which is the proof that you were employed all those years. They asked me tax from 2004 to 2007 Certified copies.
If you live in country where you don't pay income tax, then I guess they won't ask you the tax docs. In US W2 are very common and you must have it once you claim that you were employed.