On the other hand, we have never had one in our house or where I grew up and I haven't "noticed" any problems.
Specifically, we have a 40 gallon gas-fired water heater on city water with 3/4" inlet and outlet to the water heater and copper plumbing throughout the house. We have 4 bathrooms and a kitchen. The city water pressure comes in at about 85 PSI.
How important is it to add a thermal expansion tank or is this one of those "if it ain't broke, don't fix it" kind of things?
Alternatively, is there enough "give" either forward through the hot water plumbing or backward through the cold water plumbing (and ultimately maybe even back into the city mains) to allow sufficient room for expansion?
If a thermal expansion tank is recommended, any suggestions on brands, models and sizes?