I think if you have never observed ich symptoms on any of your fish, including new additions, and have adequately QT the latter then, yes, it not unreasonable to assume your tank is ich free.
In my case, I bought a tank off craigslist, including a pair of hippo tangs. I didn't do an initial fallowing of the tank, but none of the fish showed any signs of ich, so I figured I was in the clear. Ran that tank symptom free for almost 7 months. Any new additions were put though a 12 week QT process; all subsequently introduced without drama. In October of last year I upgraded to a larger tank, moving all of my fish over, though rehomed one of the hippos. Shortly thereafter, the hippo started to show some spots, though none of the other fish have. That was almost 7 months ago. The only other fish to show any symptoms is an Achilles tang that I already had in QT. At the moment, the hippo has been clear for over a month and the AT has just a few spots.
What to think? I really have no desire to remove all my fash, though accept that might be necessary at some point. Am I fortunate to have a mild strain of ich? Who knows. I feed my fish VERY heavily, and am a long experienced reefer. Does that help to manage ich? Who knows. I have a ton of SPS, zoos, etc; does that help to reduce the settling places for ich? Who knows.
