I did it once to a client who agreed to my standard rate and then tried to squeexe me down. It felt vaguely unethical. At the time he was threatening to give the work to someone who was willing to do it for $35/hr. There was no way I could bill 120 hours/week, so I called his bluff. The other guy never materialized.
OTOH car dealers do it all the time. They have a standard estimate from the manufacturer, but if they get it done sooner, you dont get a lower price.