There may be a couple of issues at work here:
1) When some people switch to E85, they also turn up turbo boost, compression or other performance factors. With higher performance usually comes a recommendation to reduce the plug heat range.
2) E85 burns cleaner than gas, so there isn't as much need to keep the plug hot. And/or a switch to E85 can sometimes mean running a bit leaner/hotter, so a cooler plug can be tolerated.
3) Some have claimed a change in plug heat range helps certain characteristics...idle, cold starting, etc.
I can't vouch for the starting as I haven't had reason to change the plug heat range...beyond running a cooler plug for the turbo. My general thought:
For a stock engine, the stock plug should be good on E85 or gas. I don't think the OEM's have any different plug for flex fuel vs gasoline - and they sure don't require a plug change when you fill up with a different fuel. So overall, as long as you aren't detonating on the hot end or carbon fouling on the cold end, the plug heat range should be OK.
For a modified engine, I would adjust the plug heat range the same as indicated for gas...ie "one heat range colder per 100hp added power" or what ever rule is prevalent in your community.
As far as starting, idling, etc - IMHO, all the plug heat range dictates is the length of the insulator in contact with the body of the spark plug. More contact means an easier path to the relatively cool cylinder head and a cooler plug. Less contact means a hotter plug. It has nothing to do with the voltage, firing characteristics, etc. So there should be minimal, if any, changes in actual start/idle characteristics. All plugs start out 'cold' in a cold engine.
There 'may' be some slight benefit due to the small electrode diameter of the platinum / iridium plugs (regardless of heat range). This requires less voltage to fire and should fire just a bit sooner which would be equivalent to a slight advance in timing over a stock plug. At the same time, there is no performance benefit in running hotter/colder plugs...you're really just trying to balance detonation and carbon fouling.
Overall, I would say run what ever plug you would run on gas. E85 has less tendency to detonate and less tendency to carbon foul. So if you are not detonating or carbon fouling, the heat range would be pretty good.