This is not a simple topic, and almost no 2 people will agree. HERE'S and old reference article from JBL on power requirements that still covers the basics.
Now days most better speakers are rated for continuous power or peak power. Some are rated for "amp power" which would correspond to peak power.
Bottom line is that you are far more likely to damage your speakers with an under-powered amp than you are with an over-powered amp. But it really depends on the source material (music) and how you use it.