I always made sure to cycle the e85 through my car at least monthly. Typically I only filled up to 1/2 a tank to avoid it being in there too long. I have seen cars that sat too long and the e85 got gooey and it was a mess. You can always test this by putting some in a jar and see what happens to it over time. I might do that myself just out of curiosity.
On the 170, I think it's a critical point that it makes full power on E65. People are already just dumping E98 in the car which not only isn't helping, but likely hurting their performance. I'd personally shoot for around E70 to E75 to keep pressure off the fuel system.