It was my understanding that the transmission in the VE didn't have overdrive, but since yours apparently does, for everyday driving you should leave it on. What it does is provides an extra gear in the transmission, lowering engine RPMs at highway speeds and increasing gas mileage. You don't really ever need to turn it off except in situations where you need extra engine breaking, such as descending a long, steep hill.
Your owner's manual should say something like this. (This is for a 2005 Corolla)
To turn “ON” or “OFF” Depress the overdrive
switch to choose either three-speed (“OFF”) or fourspeed
(“ON”) operation. During normal driving
conditions, always drive with overdrive “ON;” this
allows the transmission to shift automatically into
four-speed operation at higher speeds, increasing
fuel economy. If the indicator light on the
instrument cluster is illuminated, overdrive is “OFF.”
Just leave it on for normal driving. The only reasons you would turn it off is if you want to keep the engine rpms up. Examples are if you're pulling a heavy trailer (unlikely with a Corolla) or if you're driving where it's really hilly and you want to make sure you have quicker acceleration at your fingertips.