if i can avoid posted areas i try to.
I had 3 plots for this year picked out, trimmed the trees where needed, dug the holes and dropped my soil mix in. all that was needed was for me to bring in the plants.
my best spot got compromised, looks like someone else found the area and wants to grow there as well, i don't even think they realize there was another grow plot just 50 yds away.
so i abandoned that spot (which i was really angry about having to do) and found a new spot.....but there are posted signs. but i happen to know who owns the property and when they would be there. but i always bring my dog just in case anybody comes up on me, little easier to talk myself out of it if I am "just walking my dog" and really i am walking the dog, she loves it, chases all the squirrels and deer, plays in the creek, its a win-win
so long story short, yes i will plant in a posted area but scope it out extra good and make sure if you do get caught you have some sort of half-assed excuse for being there.