If the car is still on OEM or a compatible coolant and there isn't junk coming out, a D/F is enough. For cars on traditional green, a cooling system flush every 5 years isn't a bad idea. I have one car on G-05 I converted over that's due for fresh coolant. I can't check easily for silicate drop out but I assume having an organic acid there helps keep the silicate level low enough not to precipitate but high enough to protect.