I try to keep the injections as far as possible, but I've never really had a problem injected in any of my muscles (bi's, tri's, calfs etc) so I have lots of options. I also like to keep the pins as small as possible, I like 25g 1 inch or 1.5 if I can get it.
If I'm injecting in to the same muscle, I try to go to a different spot, which is easy with quads and glutes.