Nowadays I don't dare do that. Not with the likelihood that they'd get attacked by "ticks". With the bunny's dense fur and undercoat I would never be able to find a tick. Much less be able to remove it successfully and completely. Or at least not until they'd attached and perhaps infected the bunny.
So posting this is a word of caution for any of you that want to put your bunny on the lawn. Personally I would not risk it. Set up a playpen inside and let them play safely and tick-free.