Recent political history from 1996-2008 would seem to bear out winnings importance as Obama, John Kerry in 2004, George W. Bush and Al Gore in 2000, and Bob Dole in 1996 notched victories and were eventually elected president or won their parties nomination. These results would seem to indicate that a victory in Iowa is an essential part of any presidential aspirant until one looks at the mixed results from 1972-92.
DEMOCRATIC WINNERS (In years Caucus held) 1992: Tom Harkin (Bill Clinton finished 3rd behind Paul Tsongas.) 1988: Richard Gephardt (Michael Dukakis finished third behind Paul Simon), 1984: WALTER MONDALE NOMINEE, 1980 JIMMY CARTER (sitting president challenged by Ted Kennedy.), 1976 : JIMMY CARTER NOMINEE (though most caucus goers remained uncommitted.) 1972: Edmund Muskie (like 1976 more caucus goers were uncommitted, eventual nominee George McGovern finished second among committing caucus goers.)
REPUBLICAN WINNERS: 1976 GERALD FORD (sitting president after Nixon resignation; defeated Ronald Regan in straw poll.) 1980 George H.W. Bush (eventual nominee Reagan was second about 2,000 votes behind) 1988 : ROBERT DOLE, VP and eventual nominee Bush was third behind Pat Robertson.)
Bottom line : While a win in Iowa is no guarantee of nomination success, a strong top three showing within the land of corn, creative writers, and smash-mouth football appears to be an absolute necessity in Presidential politics.