It may actually be my ignorance - but what I'm referring to is when the keys are 'knocked in' to expand the stretchers, doesn't the dimension of the canvas (the face of the canvas) change - get bigger? It may not be enough to worry about - I don't know.
When you say stapling the corners on each side, are you referring to stapling the wooden bars at the joints - and on both the front and back? And what are you measuring on the keys?
This is all very interesting - to me at least - not so much my wife! 
And yes, I know of UCS. I actually live about two hours away from them and used to live in Owen Sound myself many years ago.
Yeah, pretty dry stuff isn't it. (The life of a photographer )
Anyway, No, the dimension of the actual canvas does not change because you fix the bars with staples to keep everything from moving.
So Yes, when you assemble the bars, you must staple the actual bars at each corner. The "Basic" bar is 7/8" deep and this bar is tongue and groove fitted, however, the pressure of stretching will actually shift the bars so its essential to staple them before you wrap a canvas around them..
The larger gallery bars (1.5 inches deep) are not tongue and groove - they are mitered corners , so they absolutely required stapling to hold them together.
You can buy tongue and groove 1.5 inch deep bars - but they are very expensive (because it increases the amount of wood and thickness of the bar ) - I don't . Its not necessary for the giclee - since the art is not really heavy (not like an artist canvas that may have a great deal of pigment splashed on it)
My mistake was using the tongue and groove and not stapling both sides (ie staple the corners on the front and turn it over and do the same thing on the back). This resulted in the key actually being able to perforate the canvas because the bars were not secured tightly.
So, given that, the dimensions of the finished canvas are very precise - and accepting you square the bars properly before stapling, the final dimension is fixed.
The keys "expand" and create tension. It essentially allows me to hand stretch , although I have found using a basic stretcher pliers makes it a bit easier, I bought from UCS - it was about $20 I think.
What I'm measuring on the key is the "pointy" end - the key is a kind of wedge shape and the smallest end fits into the bar. The problem I've had once or twice is they cut the key less than 5/8" at the smallest end - and if this happens, the key will perforate the canvas . Not good - however, I should state, that his quality is very good.
For me, having a bar that won't warp, and will adjust with keys is a real advantage. I add a dab of glue to the key AFTER I insert it to keep it from falling out, and if and when the canvas needs to be re-tensioned, the key can be easily tapped loose.
His middle "struts" are also keyed which on a long pano for example, really makes it drum tight.
I admit that if people want cheap canvas, then all of this seems over kill and I suppose its price point that decides what quality of bar you use.
To summarize my long narrative:
- first you assemble the bars
- then you measure diagonal (both ways) to ensure they are square
- then staple twice each corner on one side - flip it over and repeat on the other
- add the strut if required (no stapling here - it simply slides in)
- wrap the canvas
- pre-measure the short end of the key to ensure it is not less than 5/8"
- insert the keys into each corner and if applicable strut - lightly tap to create sufficient tension
- add a tiny dab of glue to the key at the shoulder to prevent it from falling out
Le voila!
