If they were really good, I mean exceptional, I would not mind tipping at all. I think it is a good practice but should be the exception not the norm.
So long as it remains a reward for exceptional service and not a normal state of affairs.
This business in the US of tipping waiters has gotten incredibly out of hand. We can't even not tip for poor service anymore without looking like douchebags, simply because we know none of them make enough money without tips.
I am one that does not tip if I get bad service. I could care less how it makes me look. As far as tipping a flight attendant? I would not tip them at all. They are not doing a lot to make my flight any better. So no reason to tip them. The price of the flight is bad enough I am not going to add more costs to me.