So long as you don't allow others to have physical access to your Mac, then you are about as safe as you can be on a computer. If you allow others to use your computer, then you should setup a managed account for this purpose. If you use your computer in public, then you should not allow auto logon. On your end, the danger is not from break-ins; the danger is from let-ins.
That said, your Mac is not the weak link in your financial transactions. The weak link is your financial institution and/or its contractors. Recently, the firm that processes debit card transactions for my bank was hacked. This type of crime is not terribly rare. There is nothing that you can do on your Mac to prevent something like this.
