I always add the extra time on. I think it is good for the wort, and I like to make sure I get everything out of the hops. What's an extra 10 minutes anyway.
As Bensiff noted, it is best to get the extract in and back to boiling before adding any finishing hops. I usually add my extract at 25 (to go), bring it back to the boil. adjust my times, then put in the Immersion Cooler at 20, and then run with the finishing hops per the recipe.