In general, the less frequently you pin a body part the better off you are. As a rule of thumb, I try not to pin the same site more than 1x/week. I only break this rule when pinning ED - which I usually only for short periods of time. And even when pinning ED, I'm still only hitting a site 1x every six days.
subcutaneous injections are as effective as intramuscular, the testosteroneis is just absorbed more slowly. those sub q injects also require rotation. injecting in the same place much of the time can cause hard lumps or extra fat deposits to develop in stomach area. scar tissue feels corky and is harder to penetrate.
Rotating glutes have been painless for me, and I only pin E3D so going L R L R etc hasn't been bad. It's just that, yeah, you have to be a bit of a contortionist to get a good push on the syringe. Quad was the easiest to inject, but for me it was super painful (and I was doing it properly).
Remember too that for some big muscles, you can inject multiple times in each muscle without hitting the same spot. I get 3 good locations out of each quad (I use slin pins though, less chance of hitting nerves).