The fair share of users use assistiveTouch to not have to go to a hardware key for home and have shortcuts at hand. This is persistent in Android too. The soft touch action on a screen vs the hard press of a hardware key offers completely different feel. Hence many users, me included choose to avoid the hardware key altogether. I have shortcuts to power off screen so I do not have to use the hard power key. However, I do not have any survey data to support these claims.
AssisitiveTouch is designed to move about on the screen, and it fades when not in use. That is because it is not intended to be a persistent element on the UI for all users. Considering these two points, trying to adjust to handle its position on screen is a overkill.
You need to design the application assuming that they do not have assistivetouch on. Since users know that assistiveTouch is on a overlay on the application, their mental model already considers this fact. At the same time, you have to keep design guidelines of apple in mind when you create interfaces.