Remember that if you over-pay your estimate, you get the difference back as a tax refund. Yes, it's an interest-free loan to the IRS, but the underpayment penalty is about 1% per month of the amount owed (0.5% per month penalty plus 8% APR interest), so unless you have an investment that pays +12% APR, it will be cheaper in the long run to over pay the estimate.
As stated, you can skip the 1/15 payment only if you file your complete return and pay in full by 1/31. It's risky to wait because you don't know if you will have all your information by then (1099-B, etc.) and sometimes, some tax forms are delayed by the IRS or Turbotax for various reasons.
