Overdrive distortion will not damage your woofers unless you overheat the voice coils, which is unlikely unless you're pushing them to the limit already.
However overdrive distortion can sound harsh through tweeters and/or cook them, because it can put a lot more energy and therefore heat into them, and their little voice coils can't dissipate that heat fast enough to keep from exceeding their thermal limits.
If your 2x15 is tweeterless, I wouldn't worry about it.
All you need to do is read Duke's post and you're set