amp rating vs. speaker rating

[67 455 Bird ragtop]

I hope some one can clear up a possible mis conception I've had for a while. I was always under the impression that your speakers should always be rated higher than your amp. But I was discussing this with a co-worker and he suggested it's just the opposite. And here was his reasoning. It sounds logical. But is it correct.

Let's say you have a speaker rated at 50 watts RMS. You should run an amp rated at between 80 and 100 watts. The raeasoning is you wil get better clear sounds at the lower output from your amp but still not over drive your speakers. Kind of makes sense. if you only run your volume/amp output at say 40% you're only sending 40 watts to your speakers but since it's down in the amps power band the distortion will be much less than running a 50 watt amp at 80% volume putting out the same 40 watts.

Opinions ???