Despite a long history with a manager you know and trust, I will never trust an entire organization to do the right thing. I've seen too many cautionary tales play out. In some cases, higher management directly overruled local management. If there's a bad scenario that could cost me more than I'm willing to lose, such as a big bonus or a key pension eligibility date, I would never tell anyone at the company I'm leaving until I were willing to be walked out that very day. If that means I give notice AFTER the key date and my stated end date is a little later then so be it. I'd rather work a few weeks longer than I might prefer, instead of a few weeks shorter than I need.
It's wonderful to have a good working relationship with your manager, but unless that manager is also the owner, you can only trust him to the extent of his authority. Higher level management can overrule the most sensible and humane plans of good managers. I've seen it too many times to ever trust that it cannot happen to me.