I have been thinking about your question TheRoyalsAndi. Unfortunately, I don't know the answer.
I think that the Princess Royal attends State Banquets for State Visits and also, most years, the reception for the Diplomatic Corps, but photos of royals, other than the Queen and her principal guests are rare on such occasions - which means we do not have photographic records of all the times or when the Princess Royal last wore a tiara.