I'm not sure which seal you are referring to as there are two. One is at the base between the toilet and the floor. This seal is almost never a problem and can be used again as long as it is intact, still flexible, and makes full contact around the perimeter. The second seal is in the bottom of the bowl (blade seal). Leave it closed, place about a cup of water in the bowl, and see if it leaks (watch it for a few minutes). If the bowl holds water, you are good to go. If not, replace the blade seal before re-mounting the toilet.