Yes, you can pay taxes with a cashier's check.

More Info: A cashier's check is a valid payment source for taxes as far as the Internal Revenue Service is concerned.  The IRS also accepts several other payment methods including credit card, electronic funds transfer, check, money order, or cash.

What Happens When You Make IRS Payments Late?

The IRS charges interest on late payments. Different late payments can trigger various penalties. If 1040 taxes are paid late, for example, interest and penalties may apply. If an installment agreement is paid more than 10 days after the due date, the entire installment agreement may default which could trigger adverse collection actions like levies and tax warrants. Simply call the IRS if payment problems are anticipated. Work out a payment plan and stick to it. This will help alleviate more serious problems in the future.    

How Long Should You Save Your Tax Returns?

Generally speaking, you should keep simple tax returns for a minimum of three years. If the return is complex or contains a claim for a loss, you should keep it at least 7 years.

It is important to note that it is not enough to simply keep the tax return. You must also keep all supporting records and documents pertaining to the return. If you were to be audited, unsupported numbers written on the return would not be sufficient to prove the accuracy of the return. You will need to be able to show how you arrived at those numbers.