Unless I'm mistaken, unless one has a lot of kids, the standard deduction is increasing more than most of those.I agree withholdings will be less if you do nothing at all based on the tax bracket changes and increase in standard deduction. However, most people are gaining money from the child tax credit increase. That won't be felt in your withholdings unless you go change it, only the Standard Deduction will be accounted for in the updated Withholding Tables.
Also, I have not seen how the withholdings will change. Currently withholdings are calculated based on the personal exemption ($4050/52 = $77.9 per exemption for a weekly pay check). The exemption is going away so there's no easy way to figure out the number of allowances needed to reach a specific amount of taxes withheld.