When did "God Bless America" become a required finishing line?

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5. reagan made it pretty much mandatory. god's been in the closing lines one way or another ever since

a simple "thank you" was generally more common prior to reagan, with carter, ford, and kennedy giving god a mention once each.

http://www.politicsdaily.com/2011/01/22/will-obamas-state-of-the-union-rise-above-the-cliches-of-a-stal/

The truth is that almost every rhetorical conceit in a State of the Union address is recycled. John Kennedy, a president who endorsed a strict separation between church and state, ended his 1962 speech by saying, "And in this high endeavor, may God watch over the United States of America." God's next cameo came in 1977 when Jerry Ford (who might have heard a sneeze in the audience) ended his address to Congress by saying, "Good night. And God bless you." Jimmy Carter concluded all his State of the Unions with a simple "thank you." In 1982 (the same year that he began a State of the Union tradition by introducing heroes from the balcony), Ronald Reagan combined the Ford and Carter endings by saying, "God bless you and thank you." By 1984, Reagan slipped into full Kate-Smith-singing-Irving-Berlin mode when he declared, "God bless you and God bless America."

No president has departed from this seventh-inning-stretch formula since then with the exception of Bill Clinton in 1999. Facing an impeachment trial in the Senate, Clinton went with this wordy coda: "Let us lift our eyes as one nation, and from the mountaintop of this American Century, look ahead to the next one, asking God's blessing on our endeavors and on our beloved country. Thank you, and good evening."