Sid,

I was taught the same but see the following note for instance talking about Josh Lucas's Dog.

My question is if native speakers prefer the Lucas's to Lucas'

???

See this little note please:

"Josh Lucas’s Dog Is More Popular Than He Is"

Earlier this week, at the launch for the Humane Society’s Stop Puppy Mills Project, we caught up with some cute celebrities and their even cuter pooches. But Josh Lucas’s dog might be too cute for his own good...

http://nymag.com/daily/intel/2009/07/josh_lucass_dog_is_more_popula.html