I've never built a wall (retaining or not) with less than a 3 inch base of process. Then again, I have yet to build on the conditions you describe.
Decorative walls, paver sidewalks and patios etc, all get minimum 3" base using processed stone and 1" of stone dust on top. Any actual retaining wall, steps etc, get 12" or more of the same process and 1" stone dust.
I would not use sand for the base, generally speaking sand is what Im trying to get out of my way. And sand is generally the reason you need to lay down processed stone.
Like said, its aesthetic so it probably wont matter. And being your own it can probably fly, but I would not sleep well if I did that at a customers house. It cant hurt to go deeper, Ill put it that way.
ETA: Pics to attest to experience;