You should pay your tax bill as soon as possible to reduce the amount of penalties and interest you may owe, especially if you did not make required estimated payments.
The only exception to this would be if you owe the IRS less than $1,000. In this case, you may send payment with your return.
Please remember that if you are arranging direct debit payments with your return, the date you choose should be several days in future from the date you file your return. The IRS generally takes between 24-48 hours to accept a return and if your payment date is before your return is accepted, they will not take your payment and you will have to make alternate payment arrangements.