First of all, I have never seen someone writing 'さようなら' in Kanji. 1) What is the difference between them? - 左様なら is kanji-form in 'goodbye'. But 然様 is such expressions as 'exactly', 'that is right'. And なら is conjunctuon or auxiliary verb. Thereforfe, 左様なら means 'if it is right' or 'if it is so'... something like that. In addition, なら is very coommon grammar even nowadays while 然様 is rarely found in either written or spoken language. Therefore, meaning of two expressions is completely different. And 然様なら is supposed to be used in Historical Drama. 2) Which would you use if you had to write it in Kanji? - I don't use either and I have never been faced with such situation that I have to do so. 3) Is there any particular situation/reason it would be best to use Kanji? Or do you ALWAYS use hiragana, and hiragana only? - I have no any idea of the first question, but I always use hiragara when I write さようなら to say 'goodbye'. However, I never say 然様なら to say 'if it is right'. I would say そうなら instead.