Traveling to USA before permesso di soggiorno ready

Hi,
My wife and I from the USA have multi-entry visas that are valid through July. We have applied for the permesso di soggiorno and have the postal receipts. We need to travel to the USA for one week in March, even before our questura appointment. We have to change planes in a Frankfurt on leaving and fly straight to Rome on the way back. Will we have any trouble traveling with just the postal receipt? I think since our visas are valid it should not be a problem, correct?
Thank you!

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