I am making a distinction between run-in and bed-in.
Straight out the box you should drive normally and avoid hard braking. The pad and disc surface need to wear into each other. That is "run in".
I know lots of internet posts suggest a sequence of progressively hard stops and then a cooling off, as soon as the brakes are mounted. That is a recipe for squeaking and shuddering brakes. I have seen a Youtube video where a big brand name mounts their newest discs and pads on a Mustang, without cleaning rust off the hubs...and then goes out and gives the brand new brakes hell, right out of the shop. I have also seen a lot of videos where buyers complain that this brand squeaks, squeals and shudders. I am drawing my own conclusions on the connection.
Zimmermann and ATE both recommend at least 300km of normal driving before any hard stops.
The idea is also to heat cycle the discs moderately, instead of giving them a shock. Steel is less fickle than many other alloys, but it is best to respect what heat cycling does to the internal structure. Allowing it to "normalize".
I have mounted countless brake discs and pads on my cars throughout the years and this method has provided good results every time
