No, you do not have to use your phone data with Propeller if you have a Wi-Fi connection.
However, limiting Propeller to Wi-Fi only may mean that the app can’t communicate with our server. This can result in the information in the app being out of date until you get back on Wi-Fi.
You can set Propeller to only try to send data when your phone is connected to Wi-Fi in your phone settings.
Curious how much data you’ll need to download Propeller? See this article for more information.