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 interpretationEdit: Went to the assessor's office and learned that our property tax is paid in arrears, won't get a solid answer fromso my pre-payment of what is due in 2018 is actually paying what was assessed in 2017, so feeling more confident that the IRS until it's too latededuction is proper.