Is sucking in your stomach bad?

No: The best way to tone up your stomach muscles is to exercise. When you suck up your muscles you are using the muscles in order to flatten your stomach.this is not harmful. In general you need a generalized exercise program beyond the abdomen itself.

There's an exception: ...to every rule, but I can't think of a particular circumstance assuming you're healthy and not recovering from abdominal surgery or trauma, or carrying a high risk pregnancy, where occasionally sucking in your abdomen would be "bad."