The majority of people who have TED symptoms requiring treatment by an ophthalmologist experience it within 18 months of their Graves' diagnosis, but TED is fairly capricious – it happens to some people before they have Graves', some people while they have Graves', some people long after they've been successfully treated for Graves' (our own snelsen from Seattle was stabilized after Graves' treatment for decades before TED occurred with her). I think the variation in percentages has to do with where you draw the line. I've heard that Graves' patients, when examined post-mortem, ALL have some evidence of TED in the tissues of their eyes. Only somewhere around 15% (someone correct me if I'm wrong, please?) will have symptoms requiring interventions and surgical corrections, and a small percentage of THOSE will have the worst symptoms.
As for whether every Graves' patient needs an ophthalmologist, well, that's the topic of some debate, but here are the basic points. Some suggest that it's wise to have a baseline appointment with an ophthalmologist in order that the doctor get a good idea of your “before” state, in other words, where your eyes sit in your face normally, so that they stand a better chance of helping you return to that state after TED. The argument against it is that odds are you may never need to go back, the doctor may stop practicing before you need them, and perhaps a good picture of your “normal” face would be enough, if it comes to that.
TED is an autoimmune disease, and symptoms can be exaggerated by anxiety and stress, so some of the best advice is not to worry too much about it. If it happens to you, it's a tough road, but at least you can be comforted by knowing that there are dedicated professionals learning more and more about TED every day, and surgical corrections are better all the time.
GDATF Online Facilitator