It’s my belief that the suggestion of having your peaks at -3 dB or -6 dB dates back to the early days of digital audio when mixes were captured on a digital audio tape or on an Alesis MasterLink two-track recorder, for example. You had to be careful not to clip the input which could easily sound bad.
Digital audio has advanced by leaps and bounds since those days so putting too much thought and effort into peaking at -6 dB or -3 dB in your DAW is more or less of a waste of time. It’s essentially all the same.
Read more about this here: https://theproaudiofiles.com/6-db-headroom-mastering-myth-explained/