If you get one online you usually lose your negotiation power.

The rate you get is typically based on what the secondary market will buy your loan for.

If you have good credit, etc, they can offer you a lower rate. Sooo.... ask for the lowest interest rate and have good credit.

If they offer you a par rate, that is the lowest rate they can offer your risk portfolio without the secondary market buying back for less than the loan value. I.e. It's the lowest rate for your risk portfolio, that keeps them in the green.

Some will offer lower rates if you pay money up front. This usually pays off if you plan to have the mortgage for a long time.