There wouldn't be an Airliner on the planet that takes off to the exact figures calculated anyway!
That applies to any and every aircraft - there are errors inherent is many aspects of weight control. That doesn't mean that we don't do our best to keep them under control. What is fair to say, though, is that the certification process gives you an aircraft which, if operated in accordance with design expectations (ie the AFM for the pilot) will provide you with a reasonably effective and safe piece of machinery. Diverge from the approved figures and you start to erode the fat. Diverge too far and you WILL end up dead. Now, do you know how far you can fiddle the figures ? Probably not ? Moreover, the discussion revolves around protocol, not departure accuracy.
W&B in detail is mostly to satisfy the regs!
Three things, I suggest, are of concern -
(a) design and operational safety and risk
(b) legal consequences if you get your bit wrong
(c) satisfying the Regs runs a distant third place.