1/2. I'm all for unique labels and forcing it in the form's validate handler. Make sure it's just checking against the other addresses for that user and not all the stored labels in general.

3. Regarding 1 billing address, I'm not sure. The thing is, someone may move or purchase using different credit cards that have different billing addresses. I guess I don't see a benefit in restricting it to just one.

4. What I'm referring to here is a fringe case. Normally, addresses will be entered by users for use on user accounts. But perhaps I want to save the store's mailing address to the table. I don't want that showing up in a user list, so I'd keep uid set to 0. For this reason, I still like the term "label" better than "location" which seems to narrow down the description of that column a little too much. In the end, it's just semantics, but you may need to store addresses for your store's mailing address, default pickup location for UPS, returned goods address, etc. and maybe these are all physically the same location but with different street2 lines for "Attn: Returns", "Attn: Billing", etc.