I shoot my ass all the damn time without a problem. Rotate where you put the pin in and try and move as much as possible. I wouldn't worry about it until you start having issues injecting.,
He probably doesn't reommend "site injects" because the compound is so painful.
Injecting into an area other then the glutes does not make it a "site inject", that's just a term we use around here. It makes no difference which muscle the steroids go into.