Thats not completely accurate, but the correct explanation is very technical. I will give it if you want, but its not important. What you want to look at is the efficiancy of the speakers you are using along with the RMS power rating. Also remember that many cheam amp companies, like Boss, over rate there amps by a lot, even the RMS is overated, sorta, they use a different RMS rating system than most. Now what you should do is pick an amp that is equal to or below the RMS Speaker rating, but high enough as not to underpower the speakers. The reason that underpowering speakers causeing distortion isnt accurate is that the amps is only producing a few watts for the majority of the time, it only goes above this durring certain peaks in the music. The efficiancy rating is the spl the speaker produces at 1 watt, so if the speakers are 90 db efficiant, that means at 1 watt the speaker is 90 db's. Thats a little simplified, but fine for what Im saying here. Some pro's who know what they are doing will give there speakers more than they are rated to handle, but they also know what they are doing, and can afford to replace blown speakers without issue. I had a system done like this once in a competitions setup, but since have gone to a simpler setup. I was running 100 watts to each individual speaker in the component set in a biamp setup. This is kinda above your question, or off topic I guess, whatever, so go with the other advice. Dont give your speakers too much power, too much power is more dangerous than too little. The only time too little is bad is if you are always listening at very loud levels. When you dont have enough power going to your speakers and its turned up the amp will clip. This creates square waves wich will cause the voice coil to heat up much faster and this will cause the damage. I cant give you a number for what too little is because it depends on what quality of amp you are using and how you listen to your music. The best quality amps will often put out much more power than they are rated for, and the people buying them often dont listen to there music at excessive levals. The people who buy cheap amps are, as a generalization, kids who do listen to there music too loud. So what kind of music will you listen to, how loud will you be listening too it, what kind of speakers are you powering, and what kind of amps are you looking at? then maybe I can give you even better help.
The easiest way to blow a speaker is to under drive it. A more powerful amp can't hurt as long as you are careful with the volume control. You will have more problems if your amp is grossly underpowered.
Ok Jay, sorry to argue, but what you say isnt actually true. The only way it is true, is in the way I described above. It really depends on the listener, but for the most part, your average listener does not listen with gross amounts of amp clipping. Excessive over use of power will blow a speaker way faster than will under powering a speaker. If you think of how an amp and speaker works, this all makes more sense. An amp is hardly ever producing full power, infact around 50-90% of the time its under a full watt of power. Remember that a speaker with a 90 db efficiancy will play 90 dbs at one watt, and thats right around your average loud listening level. I say average as kids today are changing that, and Im one of em, hehe. If what you said was true, then every time your amp was up or near its full output, you would be doing damage, and this isnt so. Damage from too little power is inacurate, its damage from clipping, from over voltage-something that happens when an amp clips, etc. None the less, as a safe rule of thumb, look at what the RMS for the speaker is, buy an amp within 10% of the speakers rating, that is the amp's RMS rating is within 10% of the RMS rating of the speaker, and you should be safe. Its not super scientific, but it works for your average Joe.