I've been known to outright use 'nitpick:' - and if it's a nitpick that matters to me personally will tend to supply a diff for them to grab. Keeping the effort required to resolve it in line with how much it matters to the codebase and/or other developers working on it tends to make people a lot happier about the suggestion.
I think for optional I'd just write "It may be worth ..." and I think I might prefer using 'Maybe:' as a tag because 'Optional:' implies to me "I'm sure this is a good idea but it's not strictly required" which is enough different to seem worth separating the two.
The FYI tag is a fantastic idea and I shall try and remember to use it in future.
