Putting it in park

You slow down and come to a stop. Put the car in park and let go of the brake. It shifts back & forth a couple times - is that bad for the car? I always step on the brake, put it in park, then hold the brake for a couple seconds so the car sits still. I always thought that the back & forth meant the car was swinging against the gears which would eventually damage the gears. Well the car is BARELY in park when my bf lets go of the brake and it really irks my soul!!! So I need to know, am I even right in my thought process?? Or is it something completely different? Or is it really not a big deal? He had a mustang and broke a gear on the freeway, not sure if that was his fault but he would always have a hard time switching gears & it would sound like metal clanking really bad