I think the "should I take a sleeping bag" question is the same as the "what are the best hiking boots" or "which backpack should I use" questions. This is a question that no one else can answer for you because it depends exclusively on your own body. I don't doubt that all of the posters have accurately reported on what feels good for them in those conditions, but they have their own unique internal heating and cooling systems. I always take a lightweight sleeping bag (850 grams), even last year arriving in Santiago in late July. But then I'm what the Spanish call "friolera", someone who gets cold easily. Only you know if that describes you too. If you're the one who always pulls out a sweater to warm up in summer air conditioning, or who needs extra layers when sitting around in the winter, then you will probably be very happy with the decision to carry a sleeping bag. If you sweat a lot and run around the house in shirt sleeves in winter, then you are likely to be fine with something much less.
And I agree with Helen's observations that almost everyone that I have seen walking the Camino, especially for longer distances, whatever the time of year, takes a sleeping bag. But that of course doesn't mean you should! Buen camino, Laurie