Property tax is considered to be assessed when the amount owed is finalized. In Colorado, for example, the mill levy in some counties hasn't been certified yet, so people are making estimated payments.
Historically, these estimated payments haven't been a problem with the IRS, but by their wording these estimated payments don't sound like they would be counted towards 2017. It's unclear if they'll stick to the more rigid interpretation of the wording or if (as long as you only claim the actual assessed amount) they'll allow it.
Contact your county's tax assessor to find out if the assessment is complete and if they accept prepayment (if it even benefits you).
I am prepaying last year's amount despite this uncertainty about IRS interpretation, won't get a solid answer from the IRS until it's too late.