IRS statement on Dec. 26, 2018

"The IRS is working to develop Withholding guidance to implement the tax reform bill signed into law on December 22.  We anticipate issuing the initial withholding guidance in January, and employers and payroll service providers will be encouraged to implement the changes in February. The IRS emphasizes this information will be designed to work with the existing Forms W-4 that employees have already filed, and no further action by taxpayers is needed at this time. 
 
Use of the new 2018 withholding guidelines will allow taxpayers to begin seeing the changes in their paychecks as early as February.  In the meantime, employers and payroll service providers should continue to use the existing 2017 withholding tables and systems".

Source: https://www.irs.gov/newsroom/irs-statement-withholding-for-2018

According to this IRS statement, employers can continue to use the current 2017 Tax Rate for 2018 paychecks right now.