It really is going to depend on shot frequency. I think if you inject more than once a week, and I am going to assume you do, then I would rotate between two different sites at least. Quads and glutes are the easiest to handle painwise I think. Now I just do HRT and inject once a week in my glutes, and so I only need glutes. I alternate which side I use and have had no scar tissue problems after doing this for 7 years straight.
If you are doing numerous shots each week, 4 or even more, then I would use more sites. I used to shoot in : bicep, quads, glutes, pecs, and delts. None of those ever caused any issues except for when I used gear that was really high concentration which means it had lots of solvent. I do not believe in site growth, and don't think you can make a particular muscle grow by injecting into it. I tried and experiment once where I only injected my left bicep during a 12 week cycle and at the end it had grown no more than my right.