I've never read this anywhere, but my experience is that if you leave grass clippings spread out to dry in the sun for a couple of days, they aren't nearly as "hot" (nitrogenous) if you then add them to the pile. I'd guess this is true for anything else, too.
(I'm coming at this from the opposite direction - I never have enough nitrogen.)
[six months later] In hindsight... I should have begged the question. If the pile doesn't show signs of too much nitrogen, don't fix what ain't broke. A pile with an good mix will get hot inside, maybe even steam, and smell like a dairy barn - a smell similar to urine. You may see a lot of it inside turn white. This is all great for the pile. If hot enough, it will even kill weed seeds in the pile. If it's situated where the smell is a problem, though, you may want to add carbon, as you thought, just to reduce the smell.
If a pile made mostly of grass has too much nitrogen, it will do one or more of:
- smell bad, especially inside, like spinach or lettuce that's been in the refrigerator too long
- turn black inside
- turn gooey/slimy inside