Dear Star Tiger,
I think firstly it is more important to develop a strong post-cult personality per Steve Hassan's advice (Combatting Cult Mind Control). A new identity outside the WT mind control program in which you can clearly make decisions and hold to a strong new worldveiw is vital. I think people need a strong and robust alternative worldview and healing period before they attempt this. Otherwise I feel you should not tell them.
By breaking news to your parents now, (assuming you are still searching for that replacement worldview, or are still in the process of healing), will only give them strength to remain in something they hold strongly to. You can't break them of their worldview if yours is weak and not presentable.
On the other hand if and when you have thoroughly replaced the WT mindset with an alternative, and perhaps adapted a new spiritual path, it is then I believe it's vital that you do tell them regardless. As the old Wathtower adage says, "where will we go to", will ring in their minds. If you have moved on in a truly believable and demonstratable way, then they will perhaps see the changes you've made and question their stronghold in the WT.
On a personal note;
My mother and father have been in the Corporation all their lives. I told them my new found faith in Christ and His Church. They took it well but I think they would'nt have had as much respect for me if I told them I'm just "out there" searching or told them their views are bogus because of xy and z, if you know what I mean.
In any event if you do decide to accept Christ in your life you would be better off telling your parents because only through Him can we be saved and not through a corporation. In this event let the heartache fall where it may. Tell them first chance. If you witness for Christ then you give them a chance also to be in paradise with Him, just as the theif on the cross confessed Christ at the moment of his death.
It's never to late. Time is short.
Peace
Maksym