Let Sleeping Dogs Lie
To avoid starting a conflict by bringing up a troubling situation from the past.

Example: Instead of reminding her best friend that she was against her new boyfriend from the beginning, Karen decided to let sleeping dogs lie.
"I finally finished the integration of your comments... I was really impressed how fast you could grasp the idea and use a more elegant way to express it."
Junjun Deng
Graduate Student
The University of Iowa