I use a Snugglesafe in the outside hutches if it goes below zero or if the bunny is not 100%, other than that, if they are used to being outside and have loads of hay and places to snuggle, then there shouldn't really be a need (especially if they have a bondfriend to snuggle with). I have found it works well if I stand the water bowl on a heated snugglesafe because it prevents it freezing.
Having said that, its ALWAYS worth having one on hand.
When I use them I generally come down about 3am to reheat it because by then it's cool if I do it when I go to bed.