prcd_PRA isn't a treatable condition as such although I think someone posted on here a while ago about supplements which had slowed down her Cocker's PRA. Not sure what supplements they were as the member didn't share that info & doesn't post anymore. It's a condition which is very variable in it's development anyway - some affected dogs go blind quite quickly whereas in others, it is a very slow progression over many years.
If each of your dogs has a parent tested clear, they will at most be carriers but will never develop the disease so there would be no point in testing unless you did have plans to breed.