Its true that you're shutdown on cycle, but HCG will maintain SOME natural (for lack of better word) testicular output of testosterone. But as I mentioned above, I think the real important output that you are shutting down on cycle (and preserving with hcg) is testicular pregnenolone. This is basically the "root hormone" of all your progestagens, androgens, glucocorticoids, and mineralcorticoids...it is the first hormone synthesized from cholesterol and a lot of the life extension folks think is the root cause of high cholesterol (which is identified as a side effect of steroid use).
Keep in mind, all my AAS knowledges comes from extrapolation from an HRT context--so basically I'm talking about low dose, lifetime cycles lol. But for a 12 week AAS cycle, I still can't see shutting down preg as a good thing, personally. And since HCG is so cheap, I don't see a reason why you wouldn't want to include it in your cycle.
I would recommend HRT doses. Keep in mind we're not trying to drive higher output than normal here--just maintain what we can. I think 250 iu 2-3x per week will be sufficient. Just do it at the same time you do your other shots.