Apps only use energy when they are actively running, and only use battery associated with cellular data when downloading or uploading. If you have a Wi-Fi connection it overrides cellular data, so it doesn't matter whether you have cellular data on or off; the app won't use cellular data and thus will not use energy. If you don't have a Wi-Fi connection and apps are turned off in cellular data then they won't be able to access the Internet when they are the active on-screen app. If you are not actively using an app it won't use any data, either Wi-Fi or cellular. So again, there is no reason to turn off cellular data if you have unlimited data.
The exceptions:
Background App Refresh: If you want to control battery use the better place is Background App Refresh. If apps are on there then they are allowed to use data when they are not the active on-screen app. Very few do, however.
Wi-Fi Assist: If this is on, when your Wi-Fi speed is slower than your cellular data speed it will switch to cellular data and not use Wi-Fi. This will mostly happen when using public Wi-Fi hotspots.
In general, it is not necessary to micromanage iOS. It does a very good job on its own.