This is correct, this way returning users won't lose their progress.
You can also set it to empty with
*if: not iterations
>> iterations = []
Also, note that there's a bug with *set for the moment that may prevent you from using it in this context; using *set below an *if in this manner can currently cause problems. (Basically, the *set will not persist outside of a nested child program that it's *set in.) This should change soon.
Using the >>...= format, as you did, is still fine.