I personally think a little bit of ''cheating" is fine, especially on the last 1-2 reps when you want to move up in weights (but it's similar to forced reps or negative reps, so it would be a 'high intensity' technique that shouldn't be used on every exercise). If you have to cheat on more than 1-2 reps, I'd say you're using too heavy a weight and should back off a bit.
But be careful HOW you cheat. The "upper body rock" cheat for bi curls that many people use (putting a sway in the upper body while doing dthe move) can be tough on the lower back and increase risk of injury. I prefer to let my legs (rather than more injury prone lower back) help cheat a bit. At the bottom of the bicep curl, I dip down into a shallow squat, then come somewhat explosively out of the squat while moving the weight up. The added momentum from the legs helps move the weight past that sticking point, but protects the back.
Another way of "cheating" the bicep curl (which is something many people do anyway without considcering it cheating, and is often shown as the form to do the move) is to let there be more movement in the elbows: moving them forward a bit on the lift rather than keeping them plastered to your sides (as Chalene has us do). I defiinitely used that bit of cheating when doing 20# DB curls on my last CLX workout, and it helped me do all 12 reps with my highest weight yet.