From "Audio Power Amplifier Design Handbook" 2nd ed. by Douglas Self page 362 final paragpraph:
"Readers of hi-fi magazines are frequently advised to leave amplifiers permanently powered for optimal performance. Unless your equipment is afflicted with truly doubtful control over its own internal workings, this is quite unnecessary. (And if it is so afflicted, personally I'd turn it off now) While there should be no real safety risk in leaving a soundly constructed power amplifier powered permanentley, I see no point and some potential risk in leaving unattended equipment powered; in Class-A mode there may of course be an impact on your electricity bill."
"Readers of hi-fi magazines are frequently advised to leave amplifiers permanently powered for optimal performance. Unless your equipment is afflicted with truly doubtful control over its own internal workings, this is quite unnecessary. (And if it is so afflicted, personally I'd turn it off now) While there should be no real safety risk in leaving a soundly constructed power amplifier powered permanentley, I see no point and some potential risk in leaving unattended equipment powered; in Class-A mode there may of course be an impact on your electricity bill."