IRS has already ruled that if they were not assessed in 2017, they cannot be paid in advance for 2018.
That may negate some, as I don't think every state does an annual assessment.
Some states post the tax bill in the middle of the year. I normally get the year's bill in the middle of January. I think my county rushed the internet posting of the bill to this week to help people like me, although I don't know whether I checked in previous Decembers.
>>IRS has already ruled that if they were not assessed in 2017, they cannot be paid in advance for 2018.<<
That makes sense, or people would be prepaying 2019, 2020, etc., also.
But I don’t think that’s what’s going on here. I suspect people who normally would pay their 2017 taxes in 2018 (and paid their 2016 taxes in 2017) are doubling down and paying their 2017 taxes this year to get the deduction. I don’t see why the IRS would stop that, since many people routinely pay their taxes at the very end of the year.
When my deductions dropped due to lower mortgage interest, for example, I was paying one year’s property tax in the following year and then paying the next year’s tax in the same year, getting two property tax payments made in the same year. That put me over the standard deduction, barely. I look forward to the simplification presented by the $24,000 standard deduction and the limits on deductible taxes. Now I can quit worrying about gaming the tax system.