A win for Cruz is essential. A win for Trump is not essential but it shortens the process.
I went back over the IOWA results for the last several years and realized one thing. Winning there does not correlate directly with winning the nomination. It does give supporters a boost of spirits, and maybe a boost in donations to the winner.
If Trump and Cruz finish 1-2, as the latest polls says, they will each gain approximately 7 +/- first round votes at the convention.
When I saw the picture of Trump with Huckabee and Santorum at his debate alternative event I had a perfect caption for after the general in November.
“The winners of the last three Iowa Caucus presidential campaigns, none of whom went on to win the nomination.”