Q. The locality box (box 20) on my w-2 is blank, even though I have information in box 18 local wages and tips. Can I leave it blank?
A. Yes, if box 19 is also blank.
Box 18 is usually left blank, in most states. But some states do have local income taxes. But, in those states, if there's an amount in box 18, there's usually an amount in box 19 (and box 20).
It's up to you to know if you are subject to a local income tax, in your state. Excellent summary of local taxes:
https://ttlc.intuit.com/questions/4094807-having-studied-some-of-the-complaints-that-different-users...
