At the rabbit conference in NY last year (I did not attend but I heard this from others who did), a rabbit nutritionist approved kale for feeding every day... take that from that what you will...
Me? I err on the side on caution. When I do buy kale, I feed it sometimes once a day for a week or twice a day for a few days, just to be safe. My bunnies love kale, so I do try to give it to them once in a while, but not all the time.
I've had problems with Red Russian and I think the Black Kale (too much did make them a bit gassy) in the past, but not the green curly kind or the hard dark green bubbly kind.
I will feed it daily if I've got at least five other veggies on the plate making it within my 20% rule of thumb range, but if I've got a lot of it and they're getting a bunch with each meal, I'll cut them off after three or four days.
These things sure aren't simple, are they?