OS X is a multiple user operational system. You can create multiple accounts in System Preferences > Users & Groups for the same computer. OS X provides an easy way to change between accounts.
iCloud accounts are made to be used by one user in the OS. As it is, one user account in OS X can have only one iCloud account configured at the same time. Why, because it syncs iCloud services all the time, I guess.
In iTunes, you also can have only one account configured, but you can login and logout from you accounts. But you can download stuff from iTunes and keep it in the iTunes and install in your iDevices.
For example, one family with 4 people, should have configured 4 User Accounts in OS X, each User account should configure its iCloud account in System Preferences > iCloud.
One of the Users account must the the Administrador account, in that accout use iTunes to configure one iCloud account, which I call as the iCloud buying account. Use only this iTunes to sync all 4 devices. Select what to sync for each device and don't enable Automatic Downloads in iTunes Preferences.
If you bought apps or media using 2 or more accounts, first download all updates for each account, only after that connect each iPhone to sync. You can turn wireless sync to make it easy.
If you must have multiple iCloud account it is fine, but I recommend you just buy apps or media using one account.
On the devices, configure iCloud for each account in your family in Settings > iCloud, to enable iCloud services for each person or iPhone. This way evebody receives emails, photos and so on, individualy.
On the Music app, use the same iCloud account you use to buy media. Open iTunes and create playlists for each user and select the right playlist you want to sync with a device. For other medias and apps, select only the files you want in that device and sync.
This is the way I use, my family has 4 people from 46 to 12 years old. We have diferent tastes but I can manage this all like I said. We have 5 iCloud accounts, one for each person and one to buy things.
I don't know if it is the best way to do it, but was the only one that worked well under our needs.
Message was edited by: Marco Brandao