Typical power settings during cruise and other phases of flight

Is it normal to have throttles set to the maximum during cruise, in
small aircraft? I always set them all the way forward because that
seems to get the best speed, and the engine parameters still stay in
the green areas, but I don't know if this is actually a good idea.
Obviously it would be a bad idea in a car, but perhaps aircraft
engines are specifically designed (?) to operate mainly at full
throttle during cruise.

In other phases of flight I use other settings: always full throttle
for take-off (the manual said so), and low settings or idle to
descend. Also if I'm not in a rush I use lower throttle settings at
cruise, as long as I (or the autopilot) don't have to keep the nose
too high to maintain altitude.

So what is the deal? Is it okay to run for several hours at full
throttle?

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