What stick said. I've used both. In general, there is nothing dictating what you're "supposed" to do - a lot of people, including myself, concern themselves with using acid-free materials, and I would say that's the closest thing to a hard-and-fast "rule", but no one will ban you or shun you if you don't, and even those of us who care exercise different levels of caution (I don't typically use make-it-acid-free spray on tickets and memorabilia, but others here wouldn't dream of not doing so.)