Yes, you should have been paying quarterly taxes being self-employed. Whether you have employees or not doesn't matter.
I'm not sure what happens when you should have been paying quarterly and you haven't been, there may be some penalties involved. I would consult a tax preparer or the IRS yourself and ask what you should do now.
If you are married and your spouse has had enough withholdings to cover what you would owe, then you do not have to pay quarterly taxes. This is what I do. My husband has a little extra withheld from his salary, so that at the end of the year that 'refund' money that we would be getting, is much much smaller because I need to 'pay' taxes on my income. Hopefully that makes sense...
