Tanguay, Volume 1, Page 390, lists the first Létourneau as David, born in 1616, marries in 1638 and buried at Château-Richer, Quebec, on the 16th of May, 1670. His son David, born in 1639, and Phillipe, born in 1658, kept the same surname spelling. His son Jean, born in 1642 and his son Jacques, born in 1668, were known with the surname spelled L'Estourneau.
I agree with Gaston that we should always defer to the last name used while the person was living. If they signed thier name with inflection markings, then we should honor their memory with that spelling. Sometimes assumptions are made incorrectly too. Just because you see an accent in someone's profile, it doesn't mean that they have the accurate spelling either. All these items should be researched to provide the best possible portrayal of our ancestors for the Wikitree audience. It's far better to research before a profile is posted. You may need to defend your profile at a later date, when someone merges your profile with anothers. Cheers!!