Nomad true...but after about 1 minute or so the temps will be heading upwards again...by 3 minutes they will be REALLY going up. If operating with a JPI or similar I would suggest 1 minute or when the temps start climbing shut everything down.
I suspect people took the general rule of about a minute to stabilise temps/reducing turbo impeller RPM and figured if I minute was good 3 was better. They misunderstood stabilising for cooling and a new 'old wives tale' was born.