I don't think 'flatlining' is as innocuous as some people might think. While it may not look threatening in the time domain, it does have an effect on the resulting waveform in the frequency domain ... and quite a serious one. Take a pure 1kHz sine wave and raise the amplitude so that it 'flat lines'. Go far enough and you'll end up with something akin to a square wave. Harmonics galore.
It is, by very definition - Clipping.
From my experience, speaker damage is most likely from two causes: excessive cone excursion and voice coil overheating. Excluding fault conditions, if you have a system which is capable of handling 300W RMS and you tend to run it under 30W, then you shouldn't have too much to worry about, IMHO.
But I didn't say you would have nothing to worry about.