Take for example this image below of a project I am working on…
The Blue background is a DIV that is inside of another DIV that wraps all that sidebar contact section together. To get the Blue BG DIV to have the round border and not have a gap because of the parents margin/padding I had to use negative margins
margin: -9px -2px 8px -6px;
So the question is, is it bad practice to use Negative Margins or Padding?

No; it’s not bad practice, so long as you’re aware of the fact you’re using negative margins, and that this necessarily ‘pulls’/’moves’ elements from their otherwise-‘normal’ position.
Why would you even worry about this?