I asked the Research Brewer at Haas here in Yakima recently at what temperature he thought dry hopping should be done. His response: " Cool will give less grassy notes. I prefer the cleaner flavor personally."
I asked the follow up question regarding his definition of cool, but I don't remember his response and I no longer have that email.
I still have questions though. I have only gotten a grassy note on one beer that I have brewed, so it is not a major issue with me. On the other side of the equation, I wonder at the optimum temperature for solubilizing hop oils into beer. I know that there are many different 'oils' in hops that have different boiling temperatures, so I'm wondering how that applies to dry hopping.
Hmm. A good question for Stan in "Ask the Experts." I'm going to copy and paste this over there.