Besides blanking the screen, sleep mode also turns the buttons on the front of the device off so they don't accidentally get pressed. So, it's a good idea to allow the reader to go into sleep mode.
Each page refresh uses a very minimal amount of power, so you wouldn't really save much at all by turning off sleep mode. (According to Sony, you should be able to get 7500 page turns out of a fully charged battery.)
The battery does use a minimal amount of power when in sleep mode to remember settings, etc. If you're not going to be using your 505 for a few weeks, you can use the "device shutdown" under advanced settings to fully shut it off. It will take quite a bit longer to boot up when you turn it on though. (There is also an option to turn off sleep mode under settings.)
Personally, I've never shut my 505 off fully. I just let it turn it off with the power switch--which just puts the 505 into sleep mode. Even after a year and a half, I still get over 2 weeks out of a single charge--and that's with reading an hour or more on it each day.