Much would depend on your workflow I would imagine. 

If you are changing screens frequently then a darkened room is definitely to be avoided as changes in brightness will cause your iris to open and close quickly to compensate and lead to eye strain.  If you are concentrating on one particular low contrast scene then perhaps it is not so important.  
It would also depend upon the image size and screen resolution. If you are editing images for the web and have a large high res. screen then the image will not fill the screen so has plenty of area around it with your preference for background in the image editor chosen.

As a general guide I would prefer a light behind the screen to give an ambient light to the background you see with peripheral vision and avoid harsh contrast.  A small halogen desk lamp playing on a neutral coloured wall behind the screen is my preference.

As Joe has touched upon above the ideal is to have your monitor calibrated in the light which the final image/print will be seen.  Some calibration devices will adjust the screen continously to compensate for ambient light intensity and colour. They assume a  colour temperature which you set as part of the calibration process as being the final viewing light.

Whatever your chosen preference for viewing, bright light is to avoided on the monitor screen as it will cause a loss of saturation and detail in what can be seen.

hth

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html