The only thing to be aware of is that the value has to be universally unique to avoid collisions with other annotation collections. (matt.garrish.ca)
However, VEP and other annotation tools can (and often by default do) report annotations for all transcripts, providing extra information that is often valuable. (genomemedicine.biomedcentral.com)
All ebooks should retain their marginalia: Any notes, highlights, or other annotations made in an ebook should stick to that license in perpetuity — unless a user purposefully erases their own notes before resale. (macworld.com)