Do a quick comparison. Look at last year's return. If your calculations result in a balance due, then you may want to consider having the extension form signed and filed by the client, rather than doing so yourself. The client's signature on the Form 4868 proves that the taxpayer was informed of the balance due, thus protecting you from a possible claim for late payment penalties and interest. Assure the taxpayer that it is perfectly acceptable to file an extension showing a large balance due.
Many taxpayers are afraid to file extensions showing a large balance due.
Keep an annual master list of your tax preparation clients. Check off the returns that have been completed. Check off the completed returns. Then you have a starting point so you can follow up on the client and remember who needs the second extension. While you're busy doing the extensions, remember, the first estimated payment for 1997 is also due.