I think the term "cheat" is the problem. When most think of a "cheat rep" they think of someone that loads up the bar way beyond his capability, throws form out the window and is an injury waiting to happen. Or at least that is what I think of when I hear the word "cheat".
I think "form bending" or something similar could better describe what you guys are talking about.
As the weight's get heavy and max effort is being used, form is a little compromised, even great strength coaches will say this. It's a matter of "bending" your form enough to get the weight moving and keep it moving, but not so much that it will cause injury, or it no longer looks like the movement you are supposed to be doing.
90deg rows or Pendlay rows are a great example. The movement itself allows for movement of the upper half of the body, (up from, parallel) as the weight's get heavy. it is up to the lifter to determine weather his movement is no longer a row 90deg to the ground, or not. If he is almost standing up, or at 45deg by the top of the lift, t is time to reset the lift, drip the weight to a point where better form can be used.
bblazer, mentioned a hitch when deadlifting to get a Pr rep, or when doing a max, just to keep the bar moving. Things like that are "form bending", rather than actual cheating.