Yes, unless they are a very old design. Old rheostat type dimmers just changed the extra power into heat right at the dimmer switch.
Newer dimmer switches do reduce the power used as you turn the light down.
The big advantage is you can change the amount of light you use to be bright for reading and reduced for just passage way lighting or to set a romantic mood.
It is nice to have a romantic mood save money for a change.
The other good news is that dimmers also extend the life of the light bulb. Those hard to reach ceiling lamps will not burn out as soon if you use a dimmer to tone them down a bit.
When a light is dimmed by 25%, the dimmer switch saves about 20% of the electricity required. When dimmed by 50%, it saves 40% of the electricity. Use just what you need and keep some money in your pocket. A good deal.
Problems with dimmers
Fluorescent lights don’t like to be dimmed, even those marketed as dimmable. When you use “dimmable” fluorescent lamps on a dimmer switch, it actually shortens the life of the lamp.
Dimmers just don’t work with most fluorescent lamps.
Dimmers are not recommended for Compact Fluorescent Lamps (CFL’s).
Looking to the future
Dimmers do not cost much and are easy to install but there is a problem ahead. The political winds are blowing so strongly in favor of compact fluorescent lamps that we may be forced replace incandescent bulbs, that do work with dimmers, with CFL’s that don’t work with dimmers.
Yes, you will save money but you loose freedom to change your lighting levels and freedom of choice.
Australia is going this way, and they are looked at as a great example for other countries to follow, including the USA.
Soon the light bulb police will come around to you house and shoot out your old style lamps. Realistically, production of incandescent lamps will grind to a halt due to economic forces before that happens.
If you have dimmers, use them to save money now. It may not be a good investment to install more. But, then there is that romantic mood idea, so go ahead and use them if you want them.
Here is a source for dimmable CFL's.