Its very important
Back in the days when my now 18 year old son wanted to be seen with me and flew in J and F we use to walk off the plane on a consistent basis with champagne or wine from the FAs. When I asked why it was stated that it was such a pleasure for them to have a passenger (much less a child) saying please, thank you and in general being polite.
I always told my son that good manners cost you nothing and their value is priceless. I also enjoyed the wine/champagne that we got....