wyteboi makes a good point that gfcis and all of there look a likes are bullshit!! However, but i will say this.
Being grounded is important!!
Everything, besides rubber has the ability to be a conductor of electicity. When you use a ground tail to your mettal box what you are allowing is for a path of excape via whatever the box is connected to (cabinet, stud, drywall, condit, whatever). So if there is a short and 110 volts of electricity are "rejected" from the appliance that then will go throught the ground and dissipate throughout the cabinet, stud, dywall, conduit, or whatever (not frying your appliance and not starting a fire). If this is not an option (meaning there is no ground at all) the 110 volts will be "rejected" from the non grounded receptical then the electrical current will "bounce" back to the appliance and eithor fry the motor or start a fire.
Now this is for a situation where say the house gets struck by lighting or a fluck voltage influx. If there is a perminate problem in your wiring you are calling the insurenance company unless you have the copper nailed into the ground, or you run a single 14 gage wire into the basement or wherever and stip 10 in and wrap it aroung a water pipe.
Hope this helps,
CC