I live in Zone 5 and we have experienced beautiful weather (usually late May and early June temps.). I know other parts of the country are experiencing the same thing. My peonies shoots are up to 18"-24" high at a minimum. Again this would be more May growth. Now our climate is expected to return: lows in the 30's with even a 24 temp. expected. My gut reaction is to cover them lightly. I don't want to be an alarmist, but I don't want to be singing the blues in May!
It's probably going to happen here in Maryland tonight and perhaps the next couple of days. The azaleas are starting to bloom, it's definitely going to shock them. I'll be making ~60 newspaper hats to cover my peonies and hostas tonight.