You should leave them blank. Unless TurboTax specifically tells you to enter a 0 for an entry, you should always leave them blank.
Note: If box 1 (wages) of your W2 is either blank or 0.00, then that will prevent you from electronically filing as the IRS does not allow e-filed W2s with no value in box 1.
[Edited 3.12.18]
