For ease of backup and my brain, i buy everything in one account and move it to their libraries (the kids - my husband wouldn't know how to sync his Nano if his life depended on it! LOL!!).
The kids and him have access to iTunes, and they build a list of what they want - but we buy it in my account.
A friend set up separate ones for her grandkids and DD and kinda wishes she hadn't. She loses track of what has been bought in what account and where is that song, and why won't it go on HIS iPod and all that.
But everyone has to ponder what will work for them.....