I can't understand why some 500K pots would read 450K or 530K in value. That isn't acceptable.
They have to be marked with 450K/530K (on the package or pot itself) if that's the true value. If I'm going to buy 500K CTS pots I expect it to be 500K and not something lower or higher. If the value isn't correct, than it should not be sold as '500K'.
in the industry, you can have some with 20% tolerance, 10 % or 1%... 1% being the most expensives.
for sound/music it's usualy better to go closer to what's should be, but in some cases, it doesn't matter.
I agree that sometimes it's driving people's nuts..