All in all, I'd say it doesn't matter too much, as long as you get them done.
If you place them before your compound movements, they will provide a good warmup so that when you jump into the squat rack, you're properly warmed up, and there is less risk of injury. However if you went hard on the accessories, you might have to sacrifice some plates. That's not necessarily a bad thing. Squatting on already tired quads and glutes is beneficial in its own right.
If you place them after the compound movements, you will help further break down muscle fibre, and that is what we want, after all.
If you place them on a different day, you get a chance to have two anabolic states per week, which is also worth keeping in mind.
It all boils down to preference, and when in doubt, vary! Try each method for, say, 3 months and track your progress and general feeling, and see what you prefer.