No, you do not. We recommend using Wifi or cell service to download the app from the Apple app store or Google Play. But, cell service or wifi is not needed to shoot photos and video on the app. We've had brides use the app in the deep woods of Canada and on islands with no wifi. The WeddingMix app just keeps your files inside of it under the My Media section of the app, until you or your guests decide to hit Upload Selected Videos.
We do recommend using Wifi to upload because we edit using the full HD files, so they are huge! Your guests can do this at any time within 3 weeks after the wedding.