A trick to consider - if you approach year end and know you are underpaid with estimated taxes - you can cure the problem by having your employer make extra withholding in the 4th Quarter to make up the shortfall. The IRS treats withholdings as made equally each quarter for the computation - not when actually withheld. Some high rollers use this to avoid the penalty by even giving a check to an employer's payroll department to add the amount to the withholdings in December.
