Personally I use the side convenience keys for something that do not have corresponding shortcuts on the homescreen (like the camera). Pressing the K to lock the device is easy.
I had the left side key as camera until last week when i noticed after taking BB out of the pouch the camera was on & the battery had gone down by a noticeable amount. I must have clipped the key as i slid it into the pouch & it stayed on. I thought the sleep mode would have come on as its an official RIM case but maybe if the camera is on it doesn't switch off.