If your toilet isn't solidly fastened to the floor, there may be trouble in your future. Any movement of the toilet damages the seal, and could cause thousands of dollars of damage to your home.
Sometimes a rocking toilet can be fixed by tightening the two bolts at the base of the toilet, called flange bolts or closet bolts. However, if your toilet is moving at all, it means that the seal is compromised and new ring needs to be installed.
There are two types of rings that can be used: wax and rubber. The wax ring provides a pliable seal but you only have one chance of placing the toilet correctly or you damage the ring and need a new one. With the rubber ring, you can reset the toilet until you get it right, and it is also less messy.