This is always an endless argument.
For some equipment, the cycling of power can be harder on the equipment than just running continuously.
For other equipment, such a tube amps where the output tubes have a limited lifespan, you are clearly better to turn it off when it is not in use. (A power output tube may have a life of 2000 hours when powered, whether playing music or not.) Tubes are expensive and it would be wasteful to unnecessarily replace them once or twice a year.
Still other equipment doesn't even really turn off when its off. These are generally low power consumption devices, but the only way to turn off a Slim Devices Squeezebox, for example, is to unplug it. Any device that uses an external transformer will use some power as long as the transformer is plugged into a live circuit.
Heat is a big enemy of electronics. If your device generates much heat, then it'd be good to turn it off. Another advantage of turning equipment off is it reduces (but does not eliminate) exposure to power line surges. Weigh that against the stress on/off cycling places on a unit.
Take a look at your system in that light and then do what makes sense to you, keeping in mind that no piece of electronics will last forever. From an energy saving standpoint, some devices use a lot when they are on (e.g. tube amps) while other devices are very low power consumption and may not even turn off. Again, examine your system and go from there.
