heat

You should leave amps on all the time. What stresses them is turning on and turning off - the surges in the system which reduces their life (I am talking solid state here - I know nothing about tubes). Many preamps and most phono stages have no on/off switch because of this.

If you have an exceptionally dusty environment, maybe get some type of air purifier, although will create noise in the background, which we all hate.

Even if turning it on and off doesn't break the amp, it will cause premature wear and sound degradation.