Disabling Auto-Lock will basically leave your screen and back lighting on until your battery dies. That by itself is a bad idea so here's where ScreenDimmer comes in.
ScreenDimmer will give you the ability to set the time you want the back light to stay on.
ScreenDimmer does a better job at saving power because you can set it to dim or turn off the back light after 5 seconds of screen inactivity. As apposed to 2 minutes without ScreenDimmer.
ScreenDimmer is one of the best Cydia Apps out there, IMHO.