It has nothing to do with the QUALITY of the device. They only provide for ONE wire under the grounding screw (and only one such terminal) as you are not supposed to rely on the device to provide the connection between grounding wires. In some cases, you're not even supposed to use the device for neutral connections.

There are a half a dozen ways listed to join ground wires. Wirenuts (provided they are approved for ground connections, which most are, they don't have to be green) is allowed.

So is the way I did it right - connect a third wire to the grounding screw and then tie all three together with a wire nut?

On a related matter, when I run the load wire to each of the the next receptacles, should I strip a short length of each of the three wires and wrap them around the corresponding screw, without breaking the wire? Or do like I mentioned above?

250.148 Continuity and Attachment of Equipment Grounding Conductors to Boxes. Where circuit conductors are spliced within a box, or terminated on equipment within or supported by a box, any equipment grounding conductor(s) associated with those circuit conductors shall be connected within the box or to the box with devices suitable for the use in accordance with 250.148(A) through (E).

Exception: The equipment grounding conductor permitted in 250.146(D) shall not be required to be connected to the other equipment grounding conductors or to the box.

(A) Connections. Connections and splices shall be made in accordance with 110.14(B) except that insulation shall not be required.

(B) Grounding Continuity. The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, luminaire, or other device fed from the box does not interfere with or interrupt the grounding continuity.

(C) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, equipment listed for grounding, or a listed grounding device.

(D) Nonmetallic Boxes. One or more equipment grounding conductors brought into a nonmetallic outlet box shall be arranged such that a connection can be made to any fitting or device in that box requiring grounding.

(E) Solder. Connections depending solely on solder shall not be used.

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http://freenec.com/T96.html

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300.13 Mechanical and Electrical Continuity — Conductors .13 Mechanical and Electrical Continuity — Conductors.

(A) General. Conductors in raceways shall be continuous between outlets, boxes, devices, and so forth. There shall be no splice or tap within a raceway unless permitted by 300.15; 368.56(A); 376.56; 378.56; 384.56; 386.56; 388.56; or 390.6.

(B) Device Removal. In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.

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http://freenec.com/T109.html

Thank you Ron and Fred for making the rules clear and explaining the reasoning behind them. But, back to my original question, maybe quality isn't the proper word, but how do the various receptacle designs vary to justify the enormous difference in price? What to professional electricians prefer (especially if time is more important than cost)? Also, I can get the cheaper devices in "pro-packs" of 10 for a 20% savings, but not the 20A Decora ones.

Other than appearance, the major difference is usually durability. Receptacles and switches take a lot of abuse at times. Some of the real cheap ones will fail under such treatment.

On the other hand, there are some receptacles that you'll rarely use or just plug a lamp into it once and leave it there. It seems that in that case, an expensive receptacle would be a waste of money.

So it really depends on how much usage you'd think the receptacle would have.

And personally, I like the cheap ones. The expensive ones (at least the ones I've tried) seem to take a lot of force to plug anything into them. I'd rather have the reasonable ones and have to possibly replace it in 10 years for another 75 cents in cost.

"The expensive ones (at least the ones I've tried) seem to take a lot of force to plug anything into them."

Seems to be a common complaint. As an electrician, most of the homeowners on new construction choose the cheap 15A receptacles, standard style. Some choose the cheapest 15A in the Decorated style, but most don't even care enough to spend the slight increase for them. Because of what people are used to, I get more people (women) rejecting a 20A receptacle because the slots don't look as nice (happy is a word I have heard often) as the 15A. Their decision is rarely about durability. Most electricians I know have come to the conclusion that most people want the cheap 15A, and realize that they can easily upgrade to a "Spec" grade for the ones that do eventually wear out or get loose enough that they won't hold the plug anymore. I tend to recommend the standard 15A receptacles for standard locations where the receptacle is no more than 16" above the floor or countertop and where the weight of the cord won't pull out the plug when the receptacle gets a little looser. But, in garages and outdoor receptacle where the cords are heavier and get drug around and pulled on, and may hang for a longer distance, I usually put in Spec grade. The spec grade will loosen up a little after a while, but usually won't let the plug fall out. If I plan on putting in 20A receptacles for some reason, I generally put in more circuits and limit the circuits to just 2 or 3 duplex receptacles and no lights... just because of their usual locations and that people perceive the circuit as being able to have more power. I always use 12 gauge wire and 20A breakers regardless of the receptacle being 15A or 20A. Not a fan of 14 gauge 15A circuits.