I have prepared about 10 estate returns with a fiscal year end but never made the 645 election. I have not ever received IRS notices that state I could not use a FYE.  I will file a 645 in order to preserve a few other advantages that might be applicable.

• A larger exemption amount ($600 versus $300 for a simple trust versus $100 for all other trusts);
• No requirement to make estimated tax payments until after the second tax year following the decedent's death;
• Deducting medical expenses paid by the trust on the decedent's final income tax return;
• A potentially longer time frame for owning S corporation stock (period of administration versus two-year period for former revocable trusts);