Poultry seems to be good at finding out what's bad for them as toxic things will be distasteful to animals. They have to taste it first though, so if you have anything that's deadly in one bite, you should definitely remove it permanently. The only toxic plant I have are lupines, and I often see young birds take a nibble, but never the older ones.
In the end it's up to you if you want to keep them away or let them figure it out on their own. There *is* of course a risk one will disregard their taste buds and get poisoned, but then again free-ranging have other risks like predator attack too.