If the central bank doesn't follow rules, who would trust it? The central bank's entire purpose is to put national trust into individual banks; both assuring investments (accounts) and establishing base (prime) loan rates.
A central bank answers directly to the government, not the judiciary. But it still answers to power, and follows established rules.
I'm not saying it shouldn't follow rules, I said the rules might be different.
A balance sheet becomes pointless if some assets are valued at today's prices, while other assets are valued at their price from 100 years ago.