But what if I share my Apple ID?

If you use your Apple ID on multiple devices to buy apps (say, if you have one central account for your, your spouse’s, and your children’s purchases), it’s best not to convert it into an iCloud account. Although your installed iCloud account is, by default, the one you’ll use to purchase music and apps with, you can still manually sign in and out of the App and iTunes Stores on your device. Instead, you should create an entirely separate account to use with iCloud.

Awesome tip from Macworld’s Getting started with iCloud, Apple’s new sync service article.