Using a protein rest isn't really neccissary for most malts. Most malts made today are modified enough to not need it.
However, when using things like Flaked wheat, oats, rye, or barley in high amounts...a protein rest can help. Some of the German malts produced today still need a protein rest because they're not as highly modified. It will help to reduce chill haze and promote a better head on the beer by breaking down larger proteins.
I always do a protein rest on anything with larger amounts of Pilsner malts or other lighter german malts. Wheat beers I don't bother with because they'll be hazy anyway.
A protein rest can be anywhere from 113-131F but most people aim for around 120-130. I prefer mine at 122. Also, 20-30 minutes is fine for most protein rests.
Make sure you don't do protein rests on highly modified malts as it will leave you with a watery beer.