In a closed system, the pressure WILL rise when you heat (expand) water. How much all depends on how much water the ET needs to accept (based on the volume of water heated and the delta T). The characteristics of that expansion are well known and quantifiable. The bigger the air volume and the lower the percentage you are compressing, the less the pressure will rise, thus, the bigger the ET you use, the less the pressure rises. IMHO, it's easy to select one large enough to keep the pressure below 80-psi. The ET manufacturer's calculators often will NOT keep it below 80-psi...they want you to end up buying a new one, and as long as it keeps the T&P from dumping water, IMHO, they feel everything is just fine. Yes, it will work, but you'll be putting more stress on things along the way. ETs are cheap, a bigger one ends up being cheaper in the long run.
But, unless you have a closed system you don't 'need' one, but likely will, as most public systems either have or will be making their customers' systems closed to help protect the supply system from being contaminated in case a hiccup occurs and pushes potentially contaminated material back into the system.