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.)
Great info in this post. I have been wondering about the two-sided printed papers. Both sides are so pretty but why is this necessary? When you put it in your album, one side is going to be hidden. What is the benefit of having both sides printed? Is it just the ability to have more options?
The benefit is to the manufacturer and retailer - less space (and less raw materials) required to make and display two different patterns. I don't mind it so much when the second side is something relatively plain like a subtle pattern in all one color that I won't feel tempted to use, but in general I'm not a fan, since if I like only one side, pricing usually means I end up paying more for the one side I like than if it were just a single-sided sheet.
I think am one of the few who just uses what I buy. I dont think about the acid free stuff at all. They never did so in the old days and those items still survived. Now we have albums with protector sheets so it'll be preserved a lot better. I tend to use cardstock as a base but sometimes the printed paper is thick enough (DCWV) and will hold itself.
Just use what ever you like as a base, there is no actual given rule....