Is social security number required when buying cashier check?

If you are bringing more than $10,000 in cash (US or foreign cash, which does not include any kind of check or electronic transfer) to a bank for any reason, including to buy a cashiers check, in one or more related transactions, the bank must file a Currency Transaction Report (CTR) which includes your Tax ID number (SSN for most people). You can see a copy of the report here:

http://www.irs.gov/pub/irs-pdf/ffc104.pdf

Since the rules about "related transactions" and evading the reporting requirements are so nebulous, banks often report somewhat smaller amount to be on the safe side.

And a bank is required to file a Suspicious Activity Report for a transaction of any size that they deem to be suspicious. They need an SSN for that, too.

Other than that, there is no law requiring them to gather SSNs for cashiers checks. However, they are free not to do business with you if you refuse to cooperate and you are free to go somewhere else if they make unreasonable requests.

There is a lot of misinformation about the reporting requirements. The reason is that banks and casinos are subject to different rules than any other type of business. You will often read on the web (and elsewhere) claims that banks must report any transaction greater than $10,000. That is not true. Only currency transactions (not checks, not money orders, not wire transfers) need to be reported, unless they are a suspicious transaction. Rules for some other types of businesses require a broader range of transactions to be reported.