A low cadence in of itself isn't going to hurt anyone. A low cadence because you are in a gear so high that you can't turn the pedals any faster without sending a kneecap into orbit...yeah, she's a no good.

The point is to find a combination of cadence and gear that allows you to pedal continuously*, maintain a fair speed and keep your heart rate below the red zone. This is a good starting point from which to build from and much better than either pedaling your butt off for a couple of minutes, sending your heart rate into the red zone while in a gear so low that you might as well be walking, then having to stop and coast to recover, or straining your knees trying to push too high of a gear and walking up the slightest inclines.

* you are allowed to coast...it's fun, especially on downhills (as long as you are not leading a group), but should be limited to those times when doing so doesn't cause you to lose all your hard-earned forward momentum.