Rebel XS; which metering mode?

I'm a budding photographer and I've just finished Bryan Peterson's "Understanding Exposure." I am however, still unsure when to use the different types of meters. I operate in fully manual mode (usually) and have just begun the habit of taking a meter reading either from the sky or from the grass (depending on the situation). I'm unsure which situations call for partial, center-weighted or Evaluative modes... I've read the manual but I need some further clarification. Thanks in advance.

Check this diagram from the first link below and you'll understand more.

http://www.all-things-photography.com/evalua​tive-metering.html

http://www.cambridgein​colour.com/tutorials/c​amera-metering.htm

http://web.canon.jp …g/enjoydslr/par​t2/2E.html

Thanks for the links. When using evaluative metering, should I always meter the sky or ground when shooting outside, then recompose?