I should also note something I think is very important about the ache and the itch.
I have had periods of time where I gained quite well and never or very rarely had the internal itch sensation.
I am not an expert, but to me going for the itch MAY be limiting your overall potential in exchange for faster short term gains. I am currently trying for the itch, but due to my current size, I would not be disappointed if it makes me plateau.
Even if I am wrong about the itch maybe limiting your long term potential, there is little doubt in my mind that going for the itch means you are pushing the envelope close to the point of a negative injury.
Also in my humble opinion, I had to reach a certain conditioning level before the ACHE became a positive PI for me. When I started, I distinctly remember getting that ache and turtling up for long periods of time, getting corded, etc.
Overall, in my humble novice opinion, the internal itch and the ache should be approached with caution. Lets face it, these are at least somewhat negative sensations, our bodies are maybe trying to tell us something.