Based on my own observations, the shape is more related to exactly how erect the penis is, which is in turn related to "how far into the process" the dog is.
That first group of pics is clearly of dogs that are quite "early" in the process - just unsheathed, maybe they've had 3-4 ejaculatory contractions - they may not have even started actively thrusting yet, and their knot hasn't engorged (if it's even out of the sheath) by any significant amount. Basically, they're "just getting started".
As time passes and his ejaculatory contractions force more and more blood into the penis, the dog gets more and more erect - the knot swells to nearly its full size in a series of pulses that match the contractions of his ejaculations, (though it's usually still "spongy", rather than rock hard) the shaft grows in both length and thickness, again in pulses, and while still spongy, is probably starting to get fairly stiff, and it becomes essentially impossible to feel the bone inside anymore. Most dogs I've been with end up looking more or less like the center pic in the second group - the tip more or less looks like what you'd get if somebody took a saw to a broom-handle and cut off a piece off of it, leaving a more-or-less flat end. Many, but not all, dogs develop what looks like a sort of "crater" in that squared-off end at about this point (pure personal guess with no science to back it - I think the crater is probably formed by the attachment of the "outer skin" of the penis to the bone inside, with the erectile tissue pushing forward - visualize reaching into a plastic bag full of water and grabbing a little pinch of the bottom of the bag, then gripping the bag around your wrist with the other hand, and lifting it off whatever surface it's sitting on - you'll get a very similar shape)
Some at this stage might have a little "nub" sticking out of the bottom edge of the tip (where the actual opening of the urethra is) that protrudes, but for many, that nub vanishes into the main body of the penis, and might only be detectable by looking for where the fluid comes out. That's usually a dog that's basically finished for the session - He has probably stopped thrusting, and he's just standing there squirting semen in gradually diminishing jets, the knot is probably nearly rock-hard, as is the shaft, and the veins are starting to show, if they aren't already clearly visible. Any "pulsing growth" in either length or thickness of the shaft has probably stopped. As far as he's concerned, he's "finished". Any further changes to the shape of his penis are probably small and slow, and all he's really doing is pumping out semen.
Give it a little while, and his penis may still be erect, and he may be having sporadic ejaculatory contractions, but there's probably very little coming out, and his ejaculatory contractions have probably slowed to one every few seconds, rather than the one every 0.8 seconds or so that's the typical frequency for an "actively ejaculating" dog - He's reached what I've always thought of as the "dribble" stage of ejaculation, since in most cases, he's not "squirting" a stream of semen with each contraction - if he's having the contractions at all anymore - He might be forming a lonesome drop at the tip that takes 5-6 contractions over the course of several seconds to reach a size where it's big enough to drip off, where earlier in the process, he was producing a solid stream of semen with every contraction. His penis has probably turned bluish/purplish white along most of the shaft, the veins are clearly visible, and the skin of his penis is stretched tight enough that it's like a piece of plastic. (He'll also probably protest, maybe even yelp or try to bite, at anything but the very lightest touch on the tip at this stage)
Once his penis "turns bright red" and starts looking "pebbly", he's DONE - let go of him! At this stage, trying to "keep playing" will probably be resented - sometimes with yelps and teeth - and he'll almost certainly be starting to get soft and trying to withdraw, and/or laying down and trying to lick it back into the sheath.
So to FINALLY answer the question, No, I don't think that the breed has much to do with the details of the shape other than the general size, and any breed-relationship to "long and slim" (sheps, danes, dobies) or "stubby and thick" (rotties, some pits) there might be. Anything different from "normal" is almost purely idiomatic - specific to an individual dog, and not something that correlates strongly enough to a breed to be worth mentioning.
heh very detailed, real nice to get some more experienced folks in and hear what they have to say. 
