The cable should be attached to the box first. That makes it easier to remove the receptacle without ungrounding the box, and allows you to use a self-grounding receptacle if you should so choose.

You must ground any electrical box, metal or plastic. For sectional metal boxes, the ground wire must first pass under the grounding screw of the box. Then the ground wire must be pigtailed to the receptacle and also to the ground wire connecting to the next box, if there is one. The reason for pigtailing is that if you should remove the device being grounded, you will still have ground continuity of that box plus all other parts of the cc't. For a one piece metal box, the ground to downstream boxes can come off a second ground screw in the box, or you can still pigtail. If you are wiring a light to an octagonal metal box, the light's ground wire gets attached to a separate ground screw in the box; not pigtailed. That way if you change the light fixture, you won't interrupt the ground. This is all CEC rules, not NEC. I'm sure NEC isn't much if at all different.

so normel, what you are saying is i can have the bare ground wire that enters the box wrapped around the ground screw on the box, then proceeed to the blue screw on the outlet and this is ok? i find that it makes connecting the wires easier and less wires with not having to run pigtails as one has to do if connected the wire to the outlet first, then make a pigtail to connect to the box.

You must ground any electrical box, metal or plastic.

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you have to ground PLASTIC boxes in Canada? Wow, and I thougt we had some odd rules.
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this is just being picky but the screw should be green (if you are in the US). If you see it as blue, I would get checked for tritanopia or don;t do electrical work when using Viagra.

I meant green. I have blue paint for the color the room will be on my mind.

Yes, plastic boxes have ground screws on them in Canada.

Wow, does plastic carry current in Canada...just being a wise A$$. Is there a practical reason for the screw other than grounding the box?

The screw on FS and all other plastic boxes is there to allow a secure pigtailing location to other downstream devices.

So, it sounds like it isn't there to ground the box itself. If there is nothing downstream does the ground still have to be attached to the box?

Not all plastic boxes come with a ground lug, in fact most don't. Those that do are for ceiling mount lights and fans, or specialized grounding requirements like isolated ground or hospital grade stuff. In a case where no downstream grounding is required, then I see no reason to use the ground lug on the plastic box. Whenever I wire an FS weatherproof box and it has a ground lug, I use it regardless of whether anything's downstream or not. I just like the added mechanical security of looping the ground wire under the ground lug first, then on to the device.