I think my favourite story about diplomacy came from an old Readers Digest "personal glimpses". I forget which King of England it was, probably Mad King George (George III). The king suddenly decided he should take up singing opera, and hired the foremost opera singer of the day to give his opinion, with a view to becoming his voice tutor. The singer did not want the job (the king was an appalling singer) and talked his way out of it by saying, "Your Majesty would have the most enthralling, commanding voice if only the upper register had the power and clarity so lacking in the lower."
It is possible, and kind, and also more effective (if you want the person to take your opinion into account) to be diplomatic. It also can save your neck!
Marg