My understanding is that even for standard modern malt, the protein rest helps with head retention and a good foam stand.
If doing a protein rest with modern malts, I would recommend keeping it short. A protein rest on top of the protein breakdown that already happened during modification can result in the opposite of the desired effect, i.e. too many proteins can be broken down past the "foam scafolding" sizes to "yeast nutrient" (amino acid-ish) sizes.
Here's what Hubert Hanghofer (sort of the Austrian John Palmer) recommends in one of his books, "Gutes Bier selbst brauen: Schritt für Schritt - mit Rezepten," which AFAIK hasn't been translated to English in full.
- Kolbach Index < 35%: 30 minutes at 122F
- Kolbach Index 35% - 38%: 20 minutes at 129F
- Kolbach Index 38% - 41%: 10 minutes at 135F
- Kolbach Index > 41%: no protein rest
(I converted Hanghofer's Celcius temps to Fahrenheit, rounding.)
Most modern malts will be in the last two categories. Some random examples:
- Briess 2-Row Brewers Malt: 42%
- Weyermann Pilsner: 36%-42.5%
- Briess Pilsen: 37%
- Briess 10L Munich: 40%
- Weyermann Munich Type I: 37%-46%
I hasten to add that Hanghofer's recommendations are,
as far as I know, not based on experimental evidence, but on his personal impressions. My gut feeling is that they may be a bit aggressive.
The American John Palmer, whose name is oddly enough John Palmer, recommends no protein rest at all for modern malts. At least he used to. I don't know if he has changed his mind.
Personally, I don't do a protein rest unless using a lot of unmalted grains.
ETA: I wanted to check what Charlie Bamforth has to say about it, but I can't find my copy of "Foam." Anyone have it?