In my experience, it takes a bit for new brakes to bed in. Shimano has a specific protocol for doing that. Basically get up to speed (15-20 MPH), apply one brake (front or rear) not both. Use moderate to heavy brake pressure to slow down. But don't stop. Get down to a walking speed. Repeat that ~20 times. Do it again for the other brake. You can certainly alternate between them.

I think the brake dust itself is a factor in having quiet brakes. And I'm sure all of this depends on the pad brand and type.

Having said all of that, very rarely (like once every 3-4 months) for no reason I know of, I'll get a bit of brake squeak just as I'm stopping. Probably just the right combination of speed, pad condition, temperatures, and brake pressure.