Looking at changing my 235/80 16E tires for a set of 245/75 16E Michelin Ribs on my 5th wheel. For those that have done it, anything I should be concerned about with the very slight difference in size...
Mounting a tire on a rim that is too narrow tends to make the vehicle follow the ruts and grooves in the road. It also makes the vehicle more sensitive to cross winds. Obviously the more narrow the rim, the greater the affect.
Whether it is a problem or not is highly dependent on the roads and the vehicle. There is no single answer that fits all situations.