Do NOT give your SIN # to clients. A Business number is all that is required and not even that. Just a receipt will do. If they get audited, their Accountant will contact you.

I had one client (my problem dcm that was such a pain in the arse on so many levels) demand my SIN#. I refused. She tried to convince me to just call her accountant and give it to him. No, sorry. Don't do it.