Must I use a Mobile Passport receipt if I create one?

The answer is No.

I did that in December 2016, didn't tell anyone, and everything worked smooth [IAD]

I was again this month (Feb 2017) in that situation [EWR], and this is what happened after I told them:

• The first Officer (at immigration, Global Entry) claimed that this is not possible. He then send me to a colleague down the line for verification.
• The second officer explained that it is not possible, then looked it up in his computer (using my passport), and informed his colleague that it is possible, to everyone's surprise. According to his explanation, my Global Entry printout did not contain the custom clearance, but instead referred to the customs clearance I submitted before, with the app.
• Both officers agreed that I absolutely need to tell this at customs, so they can look at the right sheet (in the app), as my Global Entry printout will not clear me through customs.
• Luggage delivery got delayed significantly, and I was really late for my connection. I mean really late (21 mins to get to a different concourse and gate).
• The guy at customs was disinterested, waved me through and turned away. I decided to put more value on catching my connection and just left.
• Nobody ever came for me, no arrests, preliminary executions, or any such happened (it's been 11 days). It seems they were able to sort it out without my interaction after all.

From what the first two officer said, it is not supposed to happen or work, but it does. It does not give you any advantage - generally Global Entry is the better thing.

I recommend to not use the app when you have Global Entry.

The information that I'm inferring from https://www.cbp.gov/travel/us-citizens/mobile-passport-control/mobile-passport-control-frequently-asked-questions seems to imply that the only information that's actually being sent when you use the app is your passport info and your responses to the questions about the items you're bringing in.

Since the administrative tasks are performed by the traveler prior to the passport control inspection, MPC reduces passport control inspection time and overall wait times.

Your identity still needs to be verified by the officer and your permission to enter has to be granted.

Once the traveler submits their customs declaration form through the app, the traveler will receive an electronic receipt with an Encrypted Quick Response (QR) code. This receipt will expire four hours after being issued. Travelers then bring their passport and mobile device with their digital bar-coded receipt to a CBP officer to finalize their inspection for entry into the United States.

If you do not use the receipt, it seems that it simply expires after four hours.