I think this is so personal and specific to each individual.
I have lots of friends who are adopted, and who are adoptive parents. Every situation is different (although you should be honest, that is the only constant). I have friends who found their birth mothers after years and years and I have friends who actually have access to the information easily and have no interest in making that call. I have a friend who felt like she was the most blessed and special person in the world because her parents "Chose her" out of all the babies in the world to pick and her brother, raised by the exact same adoptive parents in the exact same environment, both adopted as babies, who said he always felt like he had been given up and why didn't his bio parents want him? Same EXACT parents, same exact upbringing, raised same way and the two children had two different takeaways.
It is all a personality thing as well.