I know I have squeaked by before with greater than 1% error without a problem.
sure, transmitting to a "forgiving" receiver.

And this of course, so many different opinions raises the question, what is the actual tolerance limit?

What the receiver will accept!

just one example:
two guys make each their '51 project and 10MHz crystals are used
one decide to err high, uses 0EBh 1,240.08 1,200 +3.34%
the other decide to err low and use 0EAh 1,183.71 1,200 -1.36%

So, now you have a compound error of 4.70% which I doubt will work when thes two try to communicate..

The sad truth is that many devices are not "true" so the tolerance you may have is not the same regardless of what you transmit to.

Thus stay true use a "UART crystal", not a "processor crystal". If you do not and can not communicate, what do you do?

While I doubt that many commercial devices are much more than 1% off, thus probaly allowing you another 1%, you do not have a leg to stand on if you are off and the other end will not accept your data. I can see no customer saying "OK my PC can communicate with the Acme device, you can not, I will call Acme and have them fix it"

I can see a beginner, who get the frequency from the processor data sheet, choosing a "processor crystal" and hoping to get by with it, but for a professional to make anything with an error of >0.25% is IMHO irresponsible.

Erik