We used to do it all sorts of ways.
Flare and smoothly roll it on was preferred AFTER initiating the flare. That way, the Nr is already a bit elevated so there's not quite as much torque excitement as the engine spools up.
Then of course we'd leave it rolled off for the touchdown auto.
If the student wasn't rolling on by 50' (and it was intended that he/she recover with power), generally we'd take control and talk about it after.
The fun one was what I like to call "you're planning on doing a full touchdown but all of a sudden the ground doesn't look quite as good as you thought it did..." or the "Uh-oh auto..."
The throttle roll was normally initiated about 15' and you allowed Nr to bleed off to about 90% as you arrested the descent. Smoothly done, doesn't exceed any limits. Some folks would crack/pull, crack/pull, but that herky-jerky stuff has always offended my sensibilities...