For taxpayer households with only one job, absolutely complete the new W-4 form for your employer. The way people are taxed has changed, and the new form will more accurately reflect your real tax situation. For uncomplicated tax situations, you should end up ‘breaking even’ (small refund or small amount owed) come tax time in 2021.
For taxpayer households with more than one job, it probably makes sense to fill out a new W-4 as well. This is particularly true for households with two jobs that earn roughly the same amount of money–your withholding will be much more accurate using the new withholding system.
If one person (taxpayer or spouse) completes the new W-4, then new W-4s should be completed for all jobs in the household. This way, each job’s withholding is calculated in the new, more accurate system.
If you like the way things are working out now, you don’t need to change, unless you get a new job after 12/31/19, or you claimed exemption from income tax withholding in 2019.
Every employee could benefit by using the IRS withholding calculator. If you enter accurate information, you should get an accurate result. Iowa has a similar online tool. For more complex scenarios, consult your tax advisor.