Same firmware across all drives when replacing?

Have a external enclosure by OWC with 4 drives in Raid 5. One drive has failed. Array is still up and readable and ready to rebuild with a replacement drive. OWC more or less said it's very important that the replacement drive have the same firmware as the others. In this case 1.5gb Seagate drives. Called Seagate for warranty replacement and they said while ideal they have customers replace drives regularly without the same firmware with no problems (as long as same make,model and size). Have a replacement drive on the way but not the same firmware due to age of originals. Any thoughts? Thank you. :)