Fn=F2 will toggle it on/off. There may be a BIOS option too: check for that. Yes turning it off saves you 5% of your battery life; a few minutes at most (those things only max out at 1.5W or so, maybe 2-4% of the overall consumption of the notebook).
G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40