I try to keep my annotations in google analytics updated when anything happens with the website it is connected to. If it's a discovery of a bug, make a note. If a bug has been fixed? Make a note. If a feature has been added... again make a note. This applies to social media postings, emails, ad campaigns. The google analytics annotation feature is perfect for looking back and seeing what was done and what effect it had.
I am truly disappointed that either this feature is not available in the mobile analytics tool, or that it is positioned in away that makes it hard to find. I guess my options are to use desktop or directly access google analytics through a mobile browser. :(