Is Prednisone a requirement for first 2 weeks?

Not sure if our cat even has FIP for a 100% fact. But Vet said that was her worry, connected with warriors 5.0 and they recommend Prednisone for the first two weeks. Not sure if our vet will even prescribe us that without asking questions, and not sure of the price. Is this something that is 100% needed when starting GS? Our cat is only having seizures and does not have a loss of appetite and is not extremely tired or exhausted.