Hayduke27 has a good point - from what I understand, once Marek's is on your land, your whole flock are potential carriers, even if they don't show symptoms. However, you could cull your whole flock, move to a new place, and find that Marek's is also present in your new place even before you arrive - that is just a chance you have to take. It's an awful decision to have to make, and I really feel for you MamaManda.
I think that as always, in the end it comes down to personal choice. If you want to breed, sell etc. then culling and starting again in a new place at least gives you a chance of starting from scratch and possibly avoiding Marek's. However, if you are happy to keep your girls as "pets with perks", and to operate a closed flock (accepting new, vaccinated birds, but never letting birds from your flock go elsewhere - keeping them until you cull them or they die of old age) then there is no reason why you should not keep your existing birds, and only euthanise when you can see that the disease is beyond treating in each individual.
I agree that it is absolutely heartbreaking to have to cull a bird, but if you are doing it because they are suffering then it is a lot easier, because you know that you are actually helping them rather than let them continue to suffer. To cull them if they are not showing active symptoms of illness is a much harder decision to make. Even if it is not something that I could do myself, I will never criticise anyone who chooses to do so - it is not an easy decision, and we all have our own methods of flock management and our own reasons for what we do.
At the end of the day, each one of us has a duty to our flock to be as informed as possible, look at our reasons for keeping chickens, and make the decision that will cause us the least heartbreak in the long run. No-one can make the decision for you, MamaManda - it is one of those life decisions that ultimately rests on your shoulders - but you should know that whatever you decide the BYC community will support you - that is what we are all here for, after all.