A computer power supply is an example of a switched-mode power supply. The power supply outputs a square-wave like pattern, with a varying frequency and duty-cycle depending on the load on the power supply. Due to the nature of these supplies, there is often a minimum load that they can supply.
While operating one of these power supplies at no-load can damage the unit, you are placing some load on it (even if it is only at 10% of it's maximum capacity). Most power supply manufacturers have a detailed power-versus-efficiency curve on their websites (example on the "Tech Specs" tab), so you can determine where your load will place you on the efficiency curve.
It would rarely make sense to increase the loading on the power supply to get a greater efficiency, as you still have to increase the power that the unit supplies. So long as you have some load on the power supply, it will be fine.
In response to your comment, I think a modern high-quality 450W power supply would be more then sufficient for your current and future needs. The Sandybridge chipset is very efficient, and I think maybe your peak power requirement might be 280W (average would be in the high 100's to low 200's for that setup). That being said, I do not think it is a problem if you go with your original 500W supply.
I would avoid going with a very low wattage power supply, unless you can find a high quality one. I can never stress the importance of getting a good high quality power supply, as that is one of the most important (and essential for long life) components you can put in a modern computer.
Finally, if you're curious, here is some extra reading material to give you an idea of how much power you can expect the computer will actually pull from the wall.