Scar tissue arises from two sources, the actual penetration of the injection and the abrasiveness of the substance injected. The first source is clear, the more shots you do, the more scar tissue regardless of what you're injecting. Using a smaller needle helps, but will not prevent it entirely.
The second source come from both solvent content and crystallization potential of the substance injected. For example, prop is especially brutal because it require a high content of BA to hold in solution (BA causes necrosis) and will crystallize within hours after injecting. The crystals tear the muscle tissue upon each contraction, which (combined with the BA) is the reason prop is painful. More tissue tearing = more scar tissue.
Rotation of injection sites does not prevent scar tissue, only evenly disperses it.