To my knowledge, the 2018 tax law changes do not affect this. But here's how I understand it, and have experienced it myself in the past.
If at the time you file your taxes, if what you owe the IRS at that time is more than $1000 or more than 10% of your total tax liability (whichever is higher), then an underpayment or under withholding penalty is assessed.
Now there are ways to get that waived. But why create more paperwork for yourself, right? You have two choices, and can even do a combination of both if you want. Either up your W-2 withholdings, or pay yourself as estimated tax directly to the IRS in the quarter the transaction occurs, or a mix of both.
But even if you do neither, so long as you are within the above mentioned limits at tax filing time, no penalties would be assessed.
