Yeah if its in a bag or pocket then yes the sensor is covered but i tend to leave it on my desk at work or at home on my coffee table.

Doing this does improve the battery life a lot for me when in standby mode! this or course goes out the window when you are using it a lot for browsing etc!

also I have noticed the standby screen percentage is quite high when doing this? its normally only 2-3% according the "battery usage"