Should we "teach the controversy" regarding if the Holocaust happened?
Yes. teach that too -very important when the truth becomes distorted according to someone's agenda. History tends to repeat itself if we live in ignorance of what mankind is capable of or try to pretend that something didn't happen that did.
Ah, I see. So you are proposing something like:
Most people including over 99% of the experts (Historians/Biologists) agree that (the Holocaust is a historical fact/the Theory of Evolution is the best explanation of the development of life), but there is a fringe element (Holocaust deniers/IDers) that claim that (the Holocaust didn't happen/the Theory of Evolution is wrong) despite all of the evidence produced to the contrary. The fringe element claims that the evidence has been misunderstood, but it has to be said that they are largely ridiculed amongst those 99% of experts.
Or perhaps you meant:
There are two competing theories: (Holocaust, no Holocaust/Theory of Evoultion, Intelligent Design) both with their supporters and detractors. It is not for us to tell you which is true. Here is the evidence for the (Holocaust/Theory of Evolution)... and here is the evidence for (no Holocaust/Intelligent Design), er wait, I'm sure it was here a minute ago...