The earlier you pay the less interest will accrue.
https://www.irs.gov/e-file-providers/paying-your-taxes
Most IRS bills you receive in the mail will list taxes, penalties and interest, and a bottom line that says "Amount due by xxx,xxxx." If you pay by that date, you will not be assessed further penalties.
If you are just now filing your tax return, and you owe a tax payment, pay it as soon as you can. You owe interest and a late payment penalty back to April 15, and if you didn't get an extension, you will owe a late-filing penalty as well. You already owe those amounts, if you don't pay them, the IRS will send you a bill. Don't worry about that now, just pay the tax as soon as you can to stop further accumulation of penalties and interest. If you do get a bill, you can apply for a waiver (abatement).
https://www.irs.gov/payments/penalty-relief