Fully agree with Icarus there Nordic.
Having done Weight & Balance for Pax & Freighters for many years, I get annoyed with the inacurracies as well. Even on the widebodied aircraft with Pcs/Wgt check in, you may assume that a full can weighs 800 or so Kg, but volume then comes into play & who knows what's in a can full of volumetric articles. One thing I'm certain of, your bosses will NEVER put in writing anything that would exempt you from fault where an incident occurs BECAUSE of an inaccurate Loadsheet. I've tried for years to get anything from my company as to my liabilities in the matter, but, to no avail.
If it is something that concerns you, all I can suggest is to have the cans weighed (or barrows for narrowbody). Yes, it's a pain in the ar$e, but I know a bigger pain. BTW. Aircraft like the 744's tell the crew how much the aircraft weighs on the stand & it's trim as well, so I think the failsafes are well & truly being developed to alleviate this problem.
Remember; it's not the accuracy of the trim, it's the thickness of the pencil!