If you file on time but underpaid estimated taxes, you only get a penalty if it’s more than than the greater of $1000 or 10%, and a variety of other conditions

If you file on time but underpaid estimated taxes, you only get a penalty if it’s more than than the greater of $1000 or 10%, and a variety of other conditions don’t apply (not tax advice, see Form 2210 for the gory details). For the average citizen, a little bit of opportunity cost is much less important than the costs of getting hit by a surprise bill, so it makes sense to err a little bit on the side of overpayment when filling out your W-4.