What Tenny said is basically correct, even if his terminology isn't perfect. I have 20 years of experience with HCG (on and off) and I have done a lot of research. HCG can increase estradiol levels by increasing the activity of testicular aromatase. It is interesting to note that AIs may not affect testicular aromatase, no one knows, in women AIs are unable to reach ovarian aromatase.
BUT there is nothing to worry about, the increase in aromatase activity is VERY slight under reasonable doses (200-500iu), I think I can tell a very slight increase in my E2 bloodwork when I'm using 500iu 2x/week.
The problem is, people used to use 5000iu at a time, this desensitizes you to HCG very quickly, even 1000iu all at once will desensitize you quickly, it's just WAY more "pseudo-LH/FSH" than the body ever makes on its own.
Another problem is, if you do completely shut yourself down on a long cycle and don't use HCG, 200-500iu won't do shit. It will take a LONG time of using 500iu 2x per week to get any ball size back at all. That being said, I don't use HCG for "ball size" it's because I want all that hormone activity going on in there, I feel it's very anabolic, if from nothing else than the slight increase in testosterone, but I think it's more than that, I FEEL better on HCG.
One more thing about HCG, I believe it is immunosuppressive. There is a lot of stuff out there talking about this, and in my experience it is definitely true. BUT you have to let it really build up in your system to feel the effect. If you do 500iu EOD, because of the long half-life, it will build up to high levels and suppress your immune system. Do 2x per week max IMO, and if you do 3x/week make sure you take 2 days off in a row for it to clear your system, and keep the dose very low (200iu mon,wed,fri, weekend off works if you have a strong immune system).
As for the original question, on cycle, I would do HCG, in PCT I might do it for the first 1-2 weeks max, then drop it, probably time it so you are dropping the HCG when your long esters are clearing your system (2-3 times half-life duration). If you blast and cruise, stay on it.
When cruising I'm currently doing 80mg cyp 2x/week subQ, and 250iu HCG 2x/week.