I mean, they already know all the info we give them anyway. My father makes the tax returns for everyone in my family, and he was off on mine by five dollars (I'll be sure to fire him for this unforgivable mistake). Something to do with tax credits for charitable donations AFAIK. The reason why I know this? Why, of course mail from the Canada Revenue Agency telling me we were five dollars off, of course!
So if they already know exactly how much we're supposed to give them or get from them, why in the world do we have to waste our time with this?