I recommend testing the tank for leaks, but if you don't find a leak, I wouldn't re-seal it. The reason is that in order to get a good seal, you need to cut out all of the old silicone from the inside of the tank so that the new silicone will be sealing to the glass, not to old silicone. Just my opinion, but glass aquariums with all silicone seals are pretty reliable if they aren't abused. You should be able to tell if they are in good shape visually.
Sonny