I think folk are getting a little muddled here. You need permission to shoot, and whether it's verbal, written or sent in smoke signals, it's still permission.
Now, I won't shoot anywhere without written permission because if something goes wrong, there is no evidence of that word of mouth agreement and the smoke signals have blown away long ago. It's not unheard of for a land owner to simply deny any knowledge of a shooter on their land if the s*** hits the fan. Whether you want to take that risk or not is up to you. But you don't legally need it in writing, strictly speaking.