When O-sensei said something like ‘Aikido is 70% atemi and 30% irimi', I believe he was addressing exactly that question. Almost all irimi movements require atemi to distract uke, as you are moving in a potentially dangerous body position without it (uke will simply react to your irimi movement instead of the atemi).
Many people mistakenly believe that that quote means that tenkan doesn't matter to Aikido (since we already have 100%), but I think it was just in answer to your exact question -- ‘when should one use atemi?'. Tenkan doesn't usually require atemi because you are already in a position away from harm.
Take kaiten-nage for instance. While the intial tenkan doesn't require atemi, as soon as you turn towards uke to duck under his arm it requires the atemi to the face in order to make that opening.
So tenkan movements (as many as I can remember this early in the morning) do not require atemi, and atemi would be superfluous.