You are far more likely to blow a sub from distortion than overpowering. It has always been my recommendation to run as much power as you can to speakers. Running the min. ammount of power to a speaker will mean you will push the amplifier much harder to move the speaker. The harder you push the amp the more distortion you will induce into the speakers. More distortion equals more damage to the speaker.
Basically. you want to run more "clean" power to your speakers to prevent them from blowing.