I agree with using a higher-octane gas. Actually it may not make a big difference on the short run, just a little difference in the mileage. But on the long run, it has a great effect on the engine.
The Octane number has some thing to do with the detonation timing or the duration of power cycle. The higher the octane number, the higher the duration of combustion. So most higher quality engines (i.e. used in German cars and other luxury cars) will have a higher stroke than standard engines. the engine size can be simply calculated as follows:
Engine Volume = n.D.S
Where n= number of cylinders
D= piston diameter
S= cylinder stroke
Since increasing the piston diameter has a limit and involves some complications at larger bores, a lot of manufacturers tend to increase the stroke to increase the engine capacity and therefore the torque. But this doesn't come for free. The longer strokes require longer combustion timing so that the power cycles will not loose the pushing power until the piston is at the top of the cylinder. In this case, using a lower octane gas will go the job specially when you don’t need a lot of torque, but think about it this way. During the power cycle, the combustion will loose the pushing power before it gets to the top of the cylinder and continue moving by the momentum. But if you need a lot of torque the piston may not get to the top easily, which affects the electronic system controlling the valves and the other mechanical components. Specially with the VTEC system, it may be critical.
So, if loading the X with big people, towing a boat or go uphill a lot, you need to consider using the 91 Octane. Otherwise, you may be affecting the life of your engine.
BTW, in San Jose, California, I noticed that some time last year the Octane numbers changed. The 91 became 90, 89 became 88 and 87 became 86. Does anyone know why?