For several years I have used a single iTunes account for purchases for my entire family. This has worked fine, but with the introduction of iCloud and iMessage it's now much "cloudier". If you don't set up separate iCloud accounts for each user/device it can introduce unintended sharing of contacts, bookmarks etc. Here are several links to sites that explain this in more detail: [more]
http://appadvice.com/appnn/2011/10/how-to-use-icloud-in-a-family-setting-share-apps-but-not-contacts