I consider it a "value-added service" to offer accurate square footage, not an unfortunate circumstance.
Since I measure almost all of my listings myself, there usually is no additional expense.
I might call an appraiser for a second opinion if the home is very irregular in layout.
I would NEVER post the tax roll's square footage, since I have no idea where they get their numbers. our county does not measure homes. They post numbers they get from other sources and they have far too many inaccuracies in the files.
As far as the homeowners' word on square footage?
Good people with good intentions often do not know the proper criteria for measuring. They may add area that cannot be included in the square footage. And some folks cannot help but puff their property a little. I tell folks that the truth is in my tape measure.
Sometimes (actually quite often) I find that the homeowner or county have underestimated the square footage. I owe it to my sellers to market all the square footage I can measure legitimately. It is fun to tell folks, "Hey, you mentioned 2300SF, but I'm coming up with 2700SF, and after calculating it three times, that is the number I am going to post."
Another upside is that while pulling the tape, I am observing the property again, from a different perspective from the listing walk-through.