@Pat271
Great question !
Personally I always make a point to respond to all reviews, either to thank them for the positives, or to address any negative points. I certainly think that it is nice to thank guests who have gone to the trouble of writing a positive review.
I am not sure whether guests actually see the review responses though unless they deliberately go looking for them. It could therefore seem to be a very nice thought to write to them separately to ensure that they see your appreciation. In terms of potential overkill it perhaps depends on the context of the overall communications that you have had with each guest, as maybe strong levels along the way of positive communication both ways, and a good review from you (which they will have seen), may already have amply covered it.
So maybe this is something just to consider on a case by case basis, perhaps where for example communications along the way and your own review have not already been sufficiently positive enough, and you have been then very warmly surprised by the strength of their review appreciation !