You can do it either way. Sanding it off and rewriting will produce good results if you have the correct pens & ink and are a reasonably good forger. If you go that route apply a coat or two of new finish, then write, then final coats of finish.
Sanding around it can be good unless the original finish had yellowed considerably or if the woods have changed colors. Some woods darken with age, others get lighter.
To test, and around the writing and then wipe the area with denatured alcohol. That will show what the area will look like with a new clear finish applied.