Mandatory as far as I'm concerned.
A friend of mine and I used to do the crosswords in the paper together but there was a subtle rule that the cryptic ones were his and the ten minute easy ones were mine, (the rest were joint property) but sometimes we strayed in to each others terrritory and there was mass swearing and insulting if we messed up the others crossword. A shakey truce was introduced with the use of a pencil with a rubber on it for messing around with the others crosswords.